There are only 2 cfm sizes for Q-jet carbs, 750 and 800. The 800 has slightly larger primary venturies. Doesn't really matter which one you use, the air-valve secondary design of this carb won't feed your engine any more air than it 'asks' for.
The Pontiac OHC-6 Sprint engines used the 750 cfm carbs.
Best book on Q-jets ever, you can learn everything about them and build up a good one with this for reference:
Rebuild-Modify-Rochester-Quadrajet-Carburetors
