Keep in mind that the Q-jet is a spread bore while the Holley DP is a square bore. That being said, a 750 Q-jet's primary opening is smaller than the Holley. While you are going down the road under light throttle, you are effectively working from a smaller carb than the same throttle on the Holley. They are not apples to apples when looking at CFM rating. That is how much they flow when the throttle is pegged. You will gain in overall power vs the 2-barrel, but your driveablitly will certainly suffer. If you want the holley, I would go to the 600cfm. Vacuum secondaries if it is an auto and/or you have highway gears, mechanical secondaries (double pump) if you have a manual or steeper gears.