cast iron, baby
if youre looking for aluminum you need to go find a 2001-2002 olds Aurora or Intrigue for the LX5 3.5l V6 or a any buick, olds, pontiac, or cadillac that had a v8 northstar. FOr any more aluminum desires, you could look at some of the newer gm offerings but i know nothing about them. Theres a very good reason for that, too.
im sure the aftermarket has aluminum heads somewhere.
after all people stick these in fieros and also race em.