I did a long thread about this a few years ago. On my last boat I did a 383 build on pair of 6.2s, it took mildly updated heads, a cam big enough to have a slight margin to protect on reversion, GLM manifolds to make 400-405 hp. There was some power left if I wanted to run premium, also if I remember when my builder tested, using a bigger throttle body had around 15 HP in it.

All told, unless you NEED to rebuild, I wouldn't do it. It didn't make the boat significantly faster. Picked up a few MPH.

Want a faster boat? Buy a faster boat.