Here's a quick way to figure the CFM an engine actually needs, and the results are going to surprise some folks:
Divide the cubic inch displacement by 2. Multiply this figure by the max RPM you plan to run. Divide this figure by 1728. The resulting figure will be the CFM the engine should theoretically be able to use, with 100% volumetric efficiency. For a more practical figure, multiply the theoretical CFM by .9, or 90% Volumetric efficiency, which is about right for a street engine without a radical camshaft and big ports and valves.
Using this formula, your 305 has a theoretical CFM of 353 at 4,000 RPM, and a practical CFM of about 318. I'd say a 750 CFM carb would be WAY over-carbureted! A 500 would be about right. If you want the best possible solution, use the factory Quadrajet calibrated for a 305, with the stock manifold for the carb. The secondary air valve prevents the carb from opening too far and leaning the mixture out, and the jetting and manifold design have had untold hours of factory dyno work built into it.
Jerry
