I own a 1968 Commander 35' Express. I will be at the boat in about 2 hours. I can measure mine if you think it would be the same as a 38'.
How close do you need to know? The prop shafts and rudders are inline. Beam is 13 so I would guess the engine/shaft and rudder centers will be 6' 6" apart.
My wife measured from the center of the rudder shaft (right at the fiberglass) to the center of the keel, approx. 30-1/4". This is not a horizontal measurement. The keel is lower than where the rudder shaft passes through the bottom of the boat.
Because the rudders are turned and she could not straighten them from outside of the boat she could not measure rudder to rudder.
I am in a leg cast and couldn't get under there to help with this.
And the answer is 5’ 2”