650 or 750 cfm. carb??

The 750 will work but it will require more tuning to get it right than a smaller carb and will provide little or no more power and will not have as good response in the rpm range where you will be driving the car. Something in the 650 to 700 range is a much better choice even for a modified motor. The only time you would really be able to use the capacity of a 750 is when you exceed 6000 rpm.

FWIW, if you check the adds in the Mopar rags for the 5.7/6.1 hemi motors that are rated for well over 400 HP they are supplied with 600 CFM carbs.