i bought a propane gas/acetalyne? torch from crappy tire, heated the area around where the undercoating is , and scraped with a 1 1/2\" putty knife. the stuff came off fairly easy, and left a slightly gooey mess that can be removed with paint thinner. i wasn't to worried about the og paint, but maybe if you're carefull, it can be saved.
be very carefull not to focus the heat on one area for too long or the metal may warp and the paint on the other side will bubble. also, focus the heat on the area around where you are scraping, not directly on it, it will come off easier and cleaner.
also, this is fire and it gets hot, so be aware of gas or oil lines or spills, and where you touch and your surroundings.
hope this helps. good luck.