Gen III's have the cam high up in the block and use SHORT pushrods. Plus the lifters are stupid beefy out of the box. My engine has a .600 lift cam and is quieter and more drivable than my old BB was with a .510 lift hydraulic flat tappet. 6.1's were stock with like .558 lift and I think 6.4's have something like .570-.580 stock. Its a whole new ball game.