600 sounds way small for 400ci. My 360 has a 670 Street Avenger and seems like it wants more, so I am going to try a 770 vacuum soon. It is harder to go too big with a vacuum since it will only take as much as the engine can handle when properly tuned (very gross simplification). Some 318's came with thermoquads capable of 750ish cfm, but the vacuum operated air valve made it possible, unlike say a mechanical double pumper.

I'm sure others will chime in, and probably all ask the same question...What is your intended usage? Driver, dragstrip, or street/strip.