If you have a slide hammer, find a nut or preferably 2that fit the threads of the hammer., tack weld the nuts together while threaded on the shaft (wet rags covering the rest of threads/shaft), then weld to the end of the nuts to a piece of left of sheet metal to the end of nuts sticking straight out. Size of the sheet metal depends on the size/shape of dent being worked, Then put several tack welds in the center of dent to start and the use the slide hammer to pull dent outward. You probably will need to take a cut off wheel to the tacks weld to move the slide hammer around some to work out a larger bent.
Kind of the same idea as the stud welder just $250+ cheaper. Slower, BUT.
Always clean out the gas tank First, and/or put exhaust of a lawn mower or similar into the tank to remove any oxygen.
DJ