I'm a dodge guy and j wouldn't even put a 318 into a boat... if you are hell bent on motor swapping a street motor into a boat stick with 350's or small block fords. But difference is in the carb, intake, cooling system, and exhaust. 4.6l and 5,0l ford motors would be the easiest. Followed by a gm 350(carb)
In all honesty though. Before you even think about swapping in a v8. But a new outdrive. Those older boats are not made to hold much power at all... a power prop and a stock v8 will rip them apart.

But seriously. Fuck boats.
Bust out another thousand.