It would help to know the hp of your 6-71TIs. They came rated up to 485hp. The prop calculator with the 485hp engines equate to a 28in.X 31in. prop. for a medium planing hull. Less hp at the shafts will equate to less pitch and possibly less dia.
This is only a rough starting point. It would also help to know what you currently have for props and current performance.