what size carb for a 400 Pontiac?

[av8or32]

My Firebird has a 400 Potniac motor/muncie 4sp combo. It has a mild cam, headers, a performer RPM manifold, and a Holley 800 double pumper carb. The problem I'm seeing is it's way too much carb for my motor. I've done the math and I keep coming up with about 600cfm. I'm planning on going to a performer intake and getting rid of the rpm. It's going to be driven on the street and maybe a track every now and then. I know I should use a vacuum secondary carb but I'm torn between the 600 and 750 e-brocks. Thoughts?