would the 600CFM work for the 383 or should i look at a 750?

Right off hand I would say 750CFM but it really would depend on the MAX RPM's that you are running.

750cfm for a 383,those annular boosters on the Summit carb should give you good throttle response at lower rpm's.When it's time to change out the L48 350ci to a 383ci I'll be using the 750cfm carb for sure...