Donut dollys... I like using things that go a bit slower and allow you to creep up on the end result where you can better watch what's happening as you go along. When I tend to use a faster method it's like sliding into an off ramp at 90 for me...
I would shy away from heat and try the donut dolly and see how that works.. Keep in mind, the optimal efficiency in using a donut dolly is when your hammer pushes the excess into the donut where the springback brings it back to flat. Too much hammer swing and you leave a dent, not enough and you're there forever. So it's a fine line to using that tool, but I find it does well for me, others haven't had the same luck but I think it's all to do with patience.
With a high crowned panel and wanting to bring a bulge down/inward, I would use a flat faced body hammer. The flat hammer works well here in pushing a convex crown inward and capturing any excess, making a smaller crown. With a fairly flat panel as you have with your roof skin there, a flat hammer on a flat panel doesn't bode well with enough panel pushed into the dolly for effective shrinking. The crowned body hammer (Snap-on BF618) on a flat panel allows you to push excess metal into the donut, just don't swing so hard you leave a dent. Start soft swings and work up as you see how it progresses. You should be able to watch any bulge decrease in height. Any stretched low (not the middle oil can) would be treated as a high from the bottom side, ie: hammer from the bottom into a donut dolly on the top. If you leave any dents you are swinging too hard. But at least make one dent so you have a benchmark to ease up from. If your patience grows thin, my next preference would be a heating tip on a dent puller. Does a better job of pin point focus than a torch or shrinking disc.
