Well I might be wrong here, but those manifolds need to go IMO. To make good power it has to breath, HEADERS. and a crossover tube (equilzer) in the dual exhaust, and like UOP said what heads do you have?? valve size, torque convertor, you really have to build a motor with a big picture in mind or it won't work the way you want it, did you have it balanced?? is this a strip motor or a street motor?? give as much facts as you can on your current build.