I cut out the old bottom of fuel tank and made a new one.

The new bottom has reinforcement inside the bottom of the tank. I made it from two strips of 2″ x 1/8″ mild steel. Each side has half the opening cut from it and is firmly stitch welded to 16ga sheet metal bottom. I welded the seam between the two places with them in place so that there would be a little extra penetration into the bottom. The welds were not pretty, but they appear to be solid. I think I had the wire speed slightly too slow, making it sputter a bit

I drilled and placed the bolts and welded the heads to the reinforcement plate then flowed solder around the bolts to seal them.

Remaining to be done is cleaning the inside of the tank of mostly light surface rust and installing the new bottom. I have not yet decided whether I will try to attach it from the inside of the tank, which will be stronger and arguably easier to seal, or the outside, which will probably be easier overall to do.