Regardless of the engine, propping is really important. I can prop a 3.0 litre engine in a 20' to be quite 'lively', and I can prop a 5.7 in an 18' to be a dog. With a 3 litre in a 20', as Scott said, many props. That said, for tow sports, bigger is generally better. But also remember, bigger engine, generally more motion-lotion. And 4.3 in a 20' is fine. (It's what I have, and it's plenty good enough)...
Chris.........