You should get a regular gold-colored Holley 650 Double Pumper if you're actually going to change carbs. Do it once, do it right.
As far as having 20hp in tuning, I got 16. But there is also margin for error between pulls due to differences in temp and so on. The AF was pretty good and totally flat even with an out of the box carb on the initial pull, the changes were very, very minor.
Wonder what the market is for the 750.
Probably good, it came off a good running car. That's usually what people want to know.