Agreed go with the 4bbl. It is definitely worth it on a boat where you are between 3500-4500 rpm much of the time. Even though it is very expensive, the Holley 4160 I linked on the first page is already set up for your engine. Yes you can buy used, "reman" but it can be risky. I would only buy from a local trusted shop, not internet/mail order. If you have one in your area, look for a classic car restoration shop, see if they can find a marine version of the 4160 and then you have to set it up for your engine. Marine carbs ARE different do not use an automotive carb on your engine.
The other option is an Edelbrock 1409, last time I checked they were a lot cheaper than Holleys, some like them better than Holleys as well, due to there being less places for them to leak gas. The Holley modular design although it is very adaptable and easy to modify, has a lot of places to leak fuel (front and rear float bowls, fuel transfer pipe, bottom mounted accel pump, etc). The Edelbrock & Rochester Quadrajet not so much. I myself an a fan of the Quadrajet, even though the last ones were produced a long time ago, you can still get just about all the parts you'd need.
