Have you folks ever tried using a handheld flanger and silver-bearing solder like Harris Stay-Brite? It melts at very low temp, works on ferrous metals just fine and is stronger than brazing. My experience has been on flat panels or simple curves (like fender bottoms) it can turn hours of work into minutes of work and it is easy to get the hang of using it. Plus all you need is the flanging tool, some sandpaper, solder and flux and a propane torch. Measure and cut the old part out carefully using a saber saw and fine metal blade, fit and flange the patch piece carefully all across/around the joint area, set in place, tack solder at ends and the middle and then run the solder joint from one side to the other. You can do compound/complex curves this same way, but you will have to use a narrow jaw flange tool for tight turns. You can easily make your own narrow flange tool from an old pair of vise grip pliers. Soldering is notably faster than welding and there is a pace at which the work behind you cools at just the right speed...and that's part of getting the hang of it. If you're careful with temperature, you can also say goodbye to warping. I should also mention solder is easily sanded and in some cases you'll only need a very thin layer of filler putty (if any) when you're finished.
