Actually, a 750cfm carb is probably just right for that engine. I have a 318 in my '72 Charger and the most gas it can use before spitting out raw gas in 600cfm. There is some kind of mathematical equation you can use to figure out exactly how much gas your engine can hold at a time but I don't remember it. If the 750 is too big, which I doubt, then just take it back and swap for 700cfm unless you plan to bore the engine.