I'm guessing here but your engines at 3,800 rpm are probalby not making any more than the 315hp the Yanmars Metal is talking about, your guess???
If that is true than his Yanmars with 1.36's should be right around 55 mph as well, give or take with smaller pitch wheels like yours.
Metal Bros - start there 1.36 and if the Yanmars can turn them easy bump up a couple inches and see what happens?
Remember this is all practical knowledge and assimilating with a gas boat. The logic is sound and proven but like most things it will take some T&E to get it right good luck!