If all the usual stuff is ok, like compresson [do leak down] cam timing, fuel supply and strong ignition, and not some limp dick weak yellow spark, check valve springs, they may be dead. If you are now sure the mechanics of the engine checks out good then throw away that intake manifold, those Performer manifolds are for low rpm use...try a RPM performer or a Stealth for a good upper rpm power. The carb may be a little small but should be ok for anything under 5K