The nicest and most powerful I've built wasnt dyno'd...lol. But, with good machining, very good parts, and good assembly, 600hp is about the highest I've ever seen. NA anyway. Running a cast $330 crank, heavy rods, and decent pistons, and sticking in main studs, will be safe well past 500. The problem is cap walk. Mopars dont have enough meat to make a 4bolt conversion bulletproof. So the best design uses better caps, mains studs, lighter internals, and a strong crank. Reduce the flex and harmonics, and you increase the HP capacity of the material. You have to build to stay away from detonation or the caps will start to flex at the 500hp level. The one I built is 2 years old in CA, has 2 bolt Program caps on #1-4 mains, Callies crank, solid roller cam, Shady Dell ported Edelbrocks, ect. It should be making in the 550hp range on pum pas. But he wouldnt dyno it for me...lol.