I don't know why you would have lead in the center of the roof, but sure wouldn't want to use heat to remove it. It sounds like there is a dent there, that was filled. If it was me, I would remove the headliner, if it has one, and try to work the dent out from the inside. If the lead is coming loose, try taking as much of it as yoou can with a sharp knife. What you can't get off that way, use a grinder, but make sure that you have protection. That includes atleast gloves and a respirator. You will have lead dust in the air, and that is a really bad thing!