Re: looking for carb for 4.3 merc
Edelbrock 600 cfm (model 1409), and edelbrock performer aluminum intake. I've typically dealt with summit on that kind of thing. I have one on the 4.3 that came out of my boat last night and have been very pleased with it... the stock jets / metering rods won't work, but the jet kit they sell for 1409 use on a 4.3 will get it close.
Depending on prop, loading, conditions, etc... my 4.3 would push my 18' checkmate to wot 50-60 mph.
Hope this is helpful...