650 should be enough and could give better throttle response on the street. Lots of 750s on these engines too, but might need extra tuning for lower rpm efficiency.
There are calculators that tell you how many cfm your engine requires, and it is typically a smaller carb than you would think.
This video is in the HOW TO ARTICLES section and has some pretty good info in it.
PICKING THE RIGHT CARBURETOR SIZE FOR YOUR MOTOR
