The six pack engine in my GTX is 708 horse on pump gas. I used a 73 block, installed cross bolted main caps, put bushings in the lifter bores, bored .030 over, 4.5 lightened stroker crank and assembly from 440 source. Solid comp roller cam, 633 lift I think, 1.6 crane rockers, ported Edelbrock RPM heads. Head flow 327 on the intake side. Port matched the intake, installed 1/2 spacers on top of the intake. Do not mess around with the top side of the manifold. Leave it as cast. Now I sent my carbs out to Gary Williams and had him work them over. I wasn't happy with the throttle response so I got a set of mechanical carbs. Sent then out, very happy. Now the stock air cleaner with a paper filter will cost you 40 horse. A K&N filter robs 22 horse. My 535 is all done by 6000 rpm. When we dynod my engine, we tried 7 different manifolds. A single 4 barrel ported made 15 horse more. A tunnel ram made the same, just pulled more rpm's. Tried a crossram, that castrated the engine.