My stock stroke 340 made 470hp @ 6000 rpm and 455 lb-ft @ 4400 rpm with the "LA" air gap manifold and a Quick Fuel SS-750-AN double pumper. That manifold is not going to hold you back at all.
If you want to make more power, I think you need to look into a roller cam. My engine has a hydraulic roller and beehive valve springs, but there isn't too much else about it that's out of the ordinary.