Get a copy of Cliff Ruggles qjet book. Will be the best money you ever spent for carb info. The late 70's qjets are preferred, and are mostly 800cfm. As noted above, the restrictions were made by flap openings, jets, etc. My latest qjet is from a 301 firebird, and has a larger primary bore than my 1972/400 qjet! Secondary bore size is the same on these carbs, it is the primary bore size that changed the CFM ratings.
Here is a quick & easy way to tell bore size, which is how we used to find the "right" 2bbl carb in a stock racing class when combing the salvage yard... find a drill bit that "just fits" inbetween the primay bore and the venturri. if that drill bit is sloppy (or wont fit) on the next carb, you will have an idea of where you are at. I will look at the drill bit sizes when I get out to the shop.
But do not worry much about size (i have heard size does not matter)... I just ran my iron head 455 on the dyno with a 750cfm "stock" qjet, and got over 400hp and 500 tq. Would have liked to run my 800cfm qjet, but was having a couple of issues.... For comparrison, we lost just a couple of hp running a 750cfm holley vac secondary and adapter plate to the stock manifold on that same dyno series.
Last edited by leeklm; 02-23-2013 at 10:19 AM.