If it's a two-piece bolt together tank that means you have easy access to the inside. I would treat it like an oil pan I had to fix made of Unobtanium (not cheap, anyway, Willys MB F-152);
Take it apart, bead blast down to clean metal, weld up the holes and big pits, and grind flush wherever able. (In the corners you just do the best you can with a Dremel or die grinder.)
Then one additional step for a tank that the oil pan didn't need - send the two tank halves out and have them hot-dip galvanized. That'll keep it in one piece for a while... ;-)
Or another trick that might be better if the welder sits for months at a time - Save the old steel tank in the barn rafters, make a mounting cradle, and go get the appropriate size polyethylene outboard boat fuel tank and a pair of quick-connects. When you aren't running the welder it's easy to take off and drain, or go use it on some other equipment.
Oh, and run stabilizer in the gas either way.
-->--