Be very careful selecting plugs for aluminum heads -- not only do they generally have a longer reach, but they're gasketed. Plugs for iron heads are tapered to seal against the block. If you put a tapered plug in an aluminum head it can embed itself into the aluminum, which is bad. Hence, gasketed plugs -- I use Autolite 3924's on my AFR's, and I'm pretty sure that most other aluminum heads use the same.