You don't have to preheat it to weld it. You can use a strong wire brush on a wheel and just try to remove a layer and get to fresh aluminum. Without the preheat the weld won't look great, but if the guy welding has any skills it won't leak. I've welded a cracked tranny while it was in a car before, no issues. It does depend on the size and location of the crack though. In all honesty if it was my car I'd try some kind of epoxy first though.