I am not a boat guy at all, but I would think that boats have much less shock loading than a car. I would assume power applications would be gentle and smooth.
When I built my 408 I was told the cast crank was good for 500 horse. I broke it on a 1-2 powershift. Ask your engine builder or machine shop, but I would think you are OK with cast. Does one of those engine counter rotate? Are the rod journals clocked different on the backwards turning one?
