Everybody seems to overstate the importance of heat treating aluminum before it's welded. I unfortunately have weld all day and trust me, on thin walled aluminum there is no need to preheat. Getting the paint off and the part clean woud be far more important. If your wanting a cosmeticly pleasing repain, don't weld it unless you're planning on repainting. You'll wind up boiling all the paint off at least 1.5" around the welded area.
Have you considered the possiblity of not being able to get the diameter of the head tube back to the original size? Sounds like you've managed to stretch it.
Good luck