15W-40 diesel oil (or 5W-40 if you use the boat a lot in cold weather) would be my first choice for a marine big block. It's not the highest zinc oil out there, but it's got plenty for the relatively mild cams in stock flat tappet engines. My 454s have been running on either Rotella or Delo 15W-40 for years, originally the conventional version, then they got switched to synthetic blend because at some point it became cheaper and easier to find. Many of the diesel oils are dual rated for use in gas engines (the most current ratings would be CK-4 / SN) and pretty much any of them are good performing oils, so within the realm of diesel oils, I'd just get what's easy to find and at a good price and use it. An xxW-40 is a suitable viscosity range for a 454 and there's no reason why a marine 454 is overly hard on oil, so realistically, anything in that viscosity range and with an appropriate amount of ZDDP for the flat tappet cam will do fine. If you want to know more than you ever dreamed about oil, do some searching on bobistheoilguy.com, there are people there that have done analysis on tons of different oils in different engines, etc. and a few actual oil chemists on the site as well. With modern oils, outside of certain applications with very specific needs (like 2 stroke Detroits), there is no good reason to run a straight weight oil. The issues with multigrades shearing badly, etc. that people worried about 50 years ago aren't really a concern now in most cases.

I just ran a few numbers through the Michigan Wheel online calculator. I assumed a few things for the calculations: 35 kts top speed, target RPM at WOT as 4400. Based on that and a 1:1 gearbox the recommended props come out as a 13x12 3 blade or a 12x13 4 blade. So unless you have a good indication that the top speed with that power should be significantly off from 35 kts then the 13x12 props are most likely at least pretty close to ideal.

For closed cooling I'd definitely go 160, maybe even warmer. I ended up putting 170* thermostats in my closed cooling Merc 454s to get the temps up higher when cruising slow (they run 170 - 175* at slow cruise now instead of 160 - 168*). Temps on plane hardly changed compared to the 160* t-stats (went from about 175 - 180* to 178 - 182*), but the temperature swing from running slow to fast is smaller now. Generally warmer is more efficient and will idle better as long as it's not hot enough to cause any issues. Only downside I've found to the warmer thermostats is a little more carb smell after shutdown (working on ventilation changes to eliminate that).

John, I can't view your profile for some reason. Provide me an email address and I can send over a copy of the service manual PDF.

The Michigan Wheel calculator gives a starting point of 17x16 for a 3 blade prop based on a 30 kt top speed, 1.5:1 gear ratio and a 340hp with a target of 4300 RPM at WOT. Dropping the RPM target to 4200 returns a size of 18x16. Ideally you want to be in the 4200 - 4400 RPM range at WOT. I'd confirm if you have enough hull clearance with the 18" props or if you should go down to 17". If the 18s fit well, I'd take them to a shop and see if they can re-pitch them. From what the calculator returns, an 18x15 or 18x16 should be appropriate, and there's a reasonable chance they'll be able to add about 2 inches of pitch to the existing props.

Personally, I wouldn't buy any of the cheap ($350-ish) rebuilt Quadrajets out there. Most of them are complete junk that are poorly cobbled together. I've seen a few that were not even close to correct for the engines they were supposed to go on, as well as some of those builders using non-marine castings with poorly modified vents, etc. Unless the casting itself is screwed, you're better off rebuilding the Qjet you already have. Or you can find a suitable used one to rebuild. Otherwise I'd strongly consider swapping to a different carb rather than getting one of the cheap remans.

Does it run hot all the time or only under heavier load (on plane)? And how hot is hot?

@Whitney Pillsbury The solar panels connect to a charge controller that uses the power available from the panels to charge the house batteries. Amount of power available depends on the panels you have installed and how much sun is hitting them, of course. Here's a picture of the panels on the forward cabin top of our 381. This is 2x 410W panels. Appearance-wise it'll depend a bit on how big the panels you use are and how high off the deck you mount them (an inch or 2 is good for airflow, they work better with some airflow to keep them cool). There also panels available with black frames and more of an all-black appearance to the face compared to the more conventional looking panels I have (which are residential house-sized panels). At some point once we replace the bimini with a hard top I'll add more solar and make some further battery and inverter upgrades which will allow us to handle cooking and hot water on solar/battery power. We're currently generator dependent for those activities when away from shore power, but everything else is covered by solar/battery power unless the weather is really bad. With good sun during the spring / summer / fall boating season we can reliably generate 3 - 3.5 kwh of power in a day. Once we factor in hot water, cooking, etc. (basically everything except HVAC) I estimate our total daily power consumption will be around 6 - 7 kwh.

An inverter is worth it if you can fit enough battery (and ideally some solar panels) to support it. It's often less efficient than using 12v appliances, but in many cases, a 120v version of an applicance is so much cheaper that the efficiency penalty is worth it. And having 120v power available all the time is just extremely convenient if your systems can support it. A proper house battery bank, inverter, and solar panels is probably the best thing we've done to our 381. A non-flybridge boat with a hard top is a good candidate for solar, as there's good space to install it.

Merc did things the same as Scott described. Port engine should be standard rotation, starboard should be reverse rotation. Both cams and distributors spin in the standard direction (cam should be gear driven on the reverse rotation engine). Firing orders are listed as follows according to the Merc book: Standard rotation: 1-8-4-3-6-5-7-2 Reverse rotation: 1-2-7-5-6-3-4-8 As another thing to check, once you get them both started, check the ignition timing. Assuming the original Thunderbolt IV ignitions are intact and have the standard V8-24 ignition module, you should show 8* BTDC at idle as long as it's idling around 800 rpm or less (depending on your idle setting you may need to check this in gear to get the RPM down). To keep the damper plates on the Velvet Drive transmissions happy, make sure your idle speed is set between 650 and 700 RPM in forward gear when tied to the dock (check with teh engines fully warmed up). It'll be higher in neutral or when moving. Getting it in spec should avoid damper plate rattle from RPM dropping too low or stalling, especially when maneuvering while the engines are still warming up. If you want more detail, I've got a PDF of the original Merc service manual for these engines, feel free to reach out if you want it.

On most carbed engines it's normal for fuel to evaporate out of the float bowls after a few days. It shouldn't take more than a couple of seconds to get fuel back into the carb though. If you're needing to crank for an unusually long time then the fuel pump may be leaking back or you may have an air leak in the fuel lines. Most mechanical fuel pumps move air quite effectively, so even starting from dry lines and filters doesn't take long. I did it after a carb rebuild and some fuel system changes over the winter. Both engines had fuel pulled through the lines, filters, and pushed up to the carbs after about 5 seconds of cranking. For my 454s with Quadrajets, typically up to about a week they'll start with a couple pumps of the throttle and a turn of the key, as there's still enough fuel to get an accelerator pump shot and have it get more fuel to the carb before it runs out after start. If they sit for 2+ weeks it'll often take a few seconds of cranking and then a throttle pump before they fire. If it's got Quadrajets, check for well plug leakage as Craig mentioned. It's much less common on the later year carbs, but I'm pretty sure 68 was before they improved on that issue.

I don't have firm measurements, but 14'7" sounds a little low to me going based on appearance and the height of my 381. Charlene's number of 15'10" is probably much closer. I expect you'd at least need to remove the wind deflector and helm console to have any chance of doing the Western Erie in a 410. Personally, if you like the 410, want to fit under that 15.5 foot bridge and aren't dead set on having a flybridge, I'd consider just stripping the flybridge off the top and having the lower helm with a nice hardtop (and plenty of room for solar panels or other stuff up top). That would get you under that bridge with room to spare, even if you mount radar up top or anything like that.

Personally, I rarely end up using OEM fluids and filters, but instead determine what I think will be most suitable for my application. For example, Mercruiser recommends straight 40 weight oil for my 454s. With modern oils, there's no reason for that, it's just an invitation for more startup wear and a harder to find oil. So instead, my engines get 15W-40 diesel oil (typically Delo or Rotella), which is perfectly suitable for their needs, easier to find, flows better at startup (especially in cool spring or fall weather), etc. And while many OEM oil filters are good, there's nothing magic about them compared to any good quality aftermarket filter. So for each engine, I typically pick a filter that's good quality and readily available and stick to that. For my engines, it's mostly been Wix filters as the options I'd like better (Fram Ultra or Purolator PureOne or Boss) don't produce a compatible filter (Purolator and Fram only produce the correct filter in their lower end lines that I don't want to use). For parts, I'll sometimes go OEM, sometimes not. Unless I'm in a rush and need something right away, I tend to do some analysis on quality and determine if there's a more suitable option than OEM, or if there's something equivalent in quality to OEM but cheaper. When buying BMW parts, I've found cases where there's an aftermarket version made by the same company that makes the OE BMW part. And the only difference between them is the price tag (about 50%) and having the BMW logo ground off when the part comes out of the mold (for the ones they sell directly instead of sending to BMW). Other times, there is no good quality aftermarket alternative or there's not enough information out there, so the OEM part is the best choice. For belts, I tend to go to Gates or Continental, as I know they reliably make good belts, and the appropriate belt from either tends to be far cheaper than the OEM belts of unknown origin. Hoses are selected from any good hose supplier based on what hose is suitable for the application. The only time I'll spend the extra on an OEM hose is if there's something unique about it (such as a molded shape that's truly necessary for proper fit) that isn't available from an aftermarket supplier. Generally I buy hose from Gates, Continental, Trident, or Shields. Of course, there's always the struggle for what the best option is when the OEM part is discontinued (such as the impellers for my Onan generator). In the case of that generator, there are 2 aftermarket impellers available. I've tried one and it was junk that broke apart just past the 1 year mark. I have yet to try the other, but I'm on my last NOS OEM impeller, so I might have to. Or, I may convert the unit to a different seawater pump setup to get away from the problem.

There's no prohibition on outlets on board, but because of ignition protection requirements, outlets in a gas engine room or fuel tank space would be a problem unless you used extremely expensive explosion proof outlets and plugs. I don't know of any other outlet setup that would meet ignition protection requirements. Outside of the engine room and fuel tank space, outlets are no problem.

Hours of use definitely are a factor, although with us boating seasonally and storing for ~5 months every winter, I don't factor engine hours into my change interval. Different water pumps on different engines are also a factor, as some pumps are harder on impellers than others. And not all pumps use impellers made from identical material either. I also don't really care to know how long my impellers could possibly last, as I don't want to ever see one fail due to age. An earlier change avoids unplanned changes, trouble with not priming after spring launch, and also avoids having to flush broken off impeller pieces out of the cooling system (although that's minor in my case, as they'd only make it to the trans cooler 6 inches from the seawater pump). You'll get more engine hours from an impeller on a boat used year round, as the impeller doesn't soak in antifreeze over the winters (which can harden them a bit) and also doesn't spend months sitting in the same position without moving.

I change mine every 2 years before the start of the season. If we were putting a ton of hours on the boat and boating year round, I'd probably change at 300 hours and assess condition to determine if I should go longer. At the 2 year mark (and after 2 winters soaking in antifreeze) the impellers are usually intact, not cracked or worn, but starting to get a bit stiff (which can cause issues with failing to prime if they suck any air). I figure if they're always in good condition when I change them, my schedule is appropriate and I should have a very low chance of needing to do an un-planned change (which is good, as the Merc pumps would be a pain to remove and open up on a hot engine). Rubber and fiberglass exhaust parts will melt down fast with a loss of water flow, especially if the engine is under any significant load. If there's an easy way to add it on your setup, a water flow or exhaust temperature alarm is a good idea. My generator has an exhaust temp sensor in the safety shutdown circuit, but I have not added exhaust temp alarms to the engines. However, my exhausts have 2 stage water lift mufflers, so in terms of noise, it's extremely obvious if water flow becomes significantly restricted or disappears. Personally, the only sudden impeller failures I've seen (on any engine or generator) have been on startup. While running I've seen loss of water flow happen due to sucking junk against the intake or other issues, but I haven't seen an impeller just come apart while it's pumping water.

His name is Xander. He normally does wear his shoes on the boat, although we'll take them off if he's going to be inside for a while, especially if we're on the boat for a few days. He's a good jumper for getting on and off the boat, but he's not careful about sliding on slippery surfaces, so he really needs the extra grip on the decks. When we first got shoes for him, we tried them at home. As soon as he realized he could run around on the hardwood floors faster without sliding into walls he decided he liked them.

We boat with a fairly large Standard Poodle. He can easily go 12 hours between trips ashore when he needs to, so getting him off the boat for potty breaks is generally not a problem. Any time we're at a dock it's easy, otherwise we plan to be anchored somewhere we can take him ashore in the dinghy (we carry one in davits on our 381 Catalina). He enjoys dinghy rides (even at night), so other than a bit of work getting him up and down (still working on a better solution for that) it's easy enough. Here are a couple of pictures of him enjoying the boat:

I agree, heat isn't really a substitute for winterization. Only reason I would heat instead of winterizing would be if I were planning to use the boat during the winter. And then I'd plan to keep the entire boat heated in some form and I'd want block heaters on the engines and generator (they wouldn't need to be very high wattage as they can run 24/7, heating up quickly isn't a requirement). Remote temperature and power monitoring would be a good idea as well, that way any issues can be noticed and dealt with before anything gets damaged.