Get thee a dent puller. Suction cups and a screw to pull outwards should do the trick.
Alternatively, you can do what I do to remove dents from my tuba (seriously.) Get thee a rare earth magnet with a 50 lb draw. Count and put large calibre ball bearings into the tank. Wrap the magnet in a polishing cloth to protect your paint, and use the magnet to pull the ball bearings against the inside of the dent. Move back and forth until dent is removed, and then remove all ball bearings from the tank.
Works like a charm on a highly tuned precision musical instrument, so I'm sure it will work on a fuel tank. Might cost more than a puller, though (but on the bright side, you're left with awesome ball bearings and a rare earth magnet capable of lifting your entire socket wrench set.)