I've heard aluminum heads on a cast iron block will cause a chemical reaction from the head gasket when the motor gets hot. I don't know if its just the ford mustangs in a certain year range but my moms stang blew both head gaskets and warped the aluminum heads when it did it. The mechanic that did her swap said the aluminum heads on the iron block caused a chemical reaction out of her head gaskets and caused them to blow because the motor got up to 260. I've had my old stock motor (iron heads) up to 300 with no blown head gaskets. anybody know if this is true?