As you probably know 4 bbls are rated at 1.5" of vacuum. So if you bolt a 750 cfm carb on your engine and at full throttle it's pulling 1.5" of vacuum then it's pulling it's rated 750 cfms through your engine. But more than likely it's gonna have a different vacuum value. So say at 1" of vacuum it probably be around 650 cfm and at 2" of vacuum be around 850 cfm.
The OP just has to measure the vacuum on a full throttle run to see if the carb is to restrictive.