I've heard a preheat of 250 degrees and to put the head in some kind of stand that will prevent it from warping. I have heard that aluminum heads will bend around the weld.

I've been learning to tig aluminum. I like the welders with a square wave power supply because it blasts the oxide layer off of the aluminum and you can see the molten pool you're working with. Other tig welders just melt the alumium underneath the oxide layer which covers the pool.