In the old days it was called hammer welding, done with Oxy-acety torch with a small tip. the weld spot is hot and expanded. Bill Hinds still uses a torch to do most of his work. the metal cools and shrinks causing distortion, To expand the metal back you Hammer on Dolly, weld, cool, hammer, weld, cool, hammer, Sometimes I have had someone else hold the dolly where my arm wouldn't reach. get some body shop dumpster panels and practice. heat shrinking , welding, hammer expansion, An english wheel domes a piece of sheetmetal by compressing it as you roll over that spot. you make that spot a microscope measure thinner, the metal is pushed outward and expands.