650 or 750 cfm. carb??

If the carb is set up correctly it won't be starved for gas regardless of what size you have. The worest case you can expect is the carb presents a slight restriction and will make a couple of horsepower less than a 750 and the car will be 0.1 sec slower.

Do your self a favor and size the carb for what the majority of your driving will be.