You need to space the water pump and crank pulleys out to fit the aftermarket aluminum heads. Mancini and Summit have various thickness spacers and water pump pulley spacers and shim kits. You'll probably have to shim the alternator bracket and alternator off of the head and water pump, too. It's one of the worst PITA's with aftermarket heads on a small block, but unfortunately, it's part of the price of admission.