i am not entirely sure. i know what you mean with the head gaskets though. my little bro has a 95 bored and stroked all aluminum motor now in his but before that he had the stock motor and the head gaskets were replaced several times. the reason for this was something about the heads and the block being different materials or some shiit like that so when the engine would heat up they would expand differently and pop the gasket.
im not sure about the 98's though. im sure ford probably fixed it but you know how they are... you can call a dealership im sure and find out. or do a websearch or something.
oh and low ball the person you are buying the
stang from and offer them 5k and say you are low on money or something. im sure they will do it

works every time