750 Edelbrock on a mild 400??

a 750 should be fine for a mild app like yours. I've seen them put up some good numbers when rodded and jetted correctly. Did you "read" your plugs? No bogging? no lean out? If all is good, all is good. If you go dual exhaust it has been my experience that somehow the reduction of backpressure leans the mixture slightly, if that happens just jet accordingly. If I'm not mistaken Edelbrock has some comprehensive info and charts on their website for their carbs...