you want to try to grind without putting a lot of heat in, that would shrink it more. I'd use a new grinding stone/disk with a square edge and use that edge to carefully take off the bead. Your not trying to get it real smooth at this point just flat enough to hammer.
No reason to heat the seem after grinding. More heat = more shrink.
you need a dolly in behind to hammer on to stretch the seam. Gonna have to open up the back if there are no access holes. I'd consider making a permanent access hole with a screw on cover so you can get in there with a paint brush. All this welding will rust away in no time if not sealed up.
The real trick to this work is to keep the heat to a minimum. Weld a spot at a time keep moving around the panel and let it cool, some people use wet rags, some an air line. Both work. So does working outside in the winter
