I think Harold might have misread your question. Aftermarket carbs are mostly square bore. Stock cast iron 4bbl is spread bore for Q-jets. You need an adaptor to put square bore carb on a stock 4bbl manifold. Edelbrock manifolds will accommodate square bore or spread bore (Q-jet). So if you want to run an aftermarket carb, it bolts right up on an Edelbrock intake. That is the advantage of an Edelbrock intake, along with the weight.

Like you said, according to what experts have written, stock cast iron intakes flow as good as the Edelbrock performer. Q-jets are great carbs but the ones around are usually pretty spent by now and require expert rebuilding and tuning to get them to run well. For amateurs like me, it's easiest to take a new Holley carb out of the box, plop it on the intake, and drive like hell.