you seem to be quite good with tools of any sort, so what about this:
I remember an old issue of Classic Bike or Classic Motorcycle showing an elder English gentleman who repaired dented motorcycle-tanks. He had a big amount of steel-balls of different sizes welded to steel rods in different sizes and shapes. He clamped the rods into a vice, inverted that steel-ball through the filler-hole into the tank, placed it at the right point (I guess that's the real magic) and knocked the dents off. Ok, you'll damage the inner lining of the tank, but that has to be renewed after cutting and welding, too ...
Kind regards, Bambi
'Find me kindness, find me beauty, find me truth ...'
(Dreamtheater, Learning to live)