I mentioned that before. Many years ago. You would drill holes down the crease. Incert sheet metal screws, and pull them with a slide hammer.. I nearly got reamed on that one. To hard to weld back up again. The only other thing I can think of is to use a palm nailer. Using a carriage bolt up against the dent, and do some off dolly work. It would be hitting the dent many times. It might drive it up, but then maybe it won't. A stud welder might not work either..