Some of us with larger cruisers would be tickled to death to see 3 mpg.
So if you're seeing a "few miles per gallon" increase, you are no doubt talking about a smaller boat.
I'm a Holley fan, so I'd have to suggest
that if budget allows, go with the Holley spread bore replacement.
Don't get me wrong, the Edelbrock is a good carburetor, I simple prefer the Holley.
That being said, I'd actually prefer to stay with the Rochester Quadrajet, and make them function correctly.
There are no new Q-jets, and have not been for years. So if you were to purchase a Q-jet, it will be a re-man'd carburetor.
There is an e-Bay seller under the name I-5 AutoRoca.
His name is Carlos.
I have reason to believe that Carlos would be good to deal with.
There is another seller "Carb C___ " on e-Bay....., his name begins with the letter "H"!
He does not take care of customer issues.
He has changed his seller name several times....
BE WARE!
I'd recommend steering clear of him.
I'd suggest that you not get overly anxious about CFM requirements.
Since we don't operate at high RPM, we typically do not require high CFM rated carburetors.
Here is a CFM calculator.
http://www.carburetion.com/calc.asp
Plug in your displacement in cubic inches, and your anticipated MAX RPM.
You'll be surprised at how little CFM will do the job.
Examples:
350 cu in @ 4k rpm requires only 344 cfm.
454 cu in @ 4k rpm requires only 446 cfm.
.