intake swap performer loss?

With a 2 bbl. carb you probably won't notice any performance loss with the Performer manifold. MAYBE with the 2 bbl. manifold, but that depends on which manifold you have. It IS the 360 2 bbl. manifold, right? That isn't a half-bad intake- kind of like a 2 bbl. version of the factory 4 bbl. manifold, only better one would be the 360 TBI manifold which could be redrilled for the Rochester and no adapter required, keeping the height down.