either is fine
Depends on the motor, if you have something "hotter" the twin 600's would be better but if it is mild vice versa.
I have 2x500's on my F-28 on a moderately worked Ford 5.0. Works great and looks absolutely awesome.
If you have 600's, use'em. The edelbrocks are vacuum secondary carbs so the 200 extra cfm shouldn't kill low end response too bad.
Good luck, and for a car seeing alot of street time the progressive linkage is a must, otherwise you get light switch-like throttle response!