Cut out the bad part and weld in replacements..
added:
what do you mean wet? can you smell gas? on the early models the floor was the top of the tank and if it got holes in it, you had to replace it.. don’t try to repair the gas tank..
I had the same problem with my ‘71. I just put a rust converter on the floor and didn’t have a problem. Didn’t even replace the few places where there were holes — ran like that for years. But I also never put the mat back in. The only way to replace the metal is to cut it out. Unless you are going after a concourse car, leave it alone and just convert and paint. In these models, the gas tank is not the floor of the trunk.
The leak originates from around the rear window and the seam welds, they are notorious for rusting under and around the gasket. Same for around the rear lights.
I would recommend you pop the rear window out and inspect carefully. Use the rust converter on the surface after you’ve scrapped the rust away and then fill all holes. Replace the window with a new gasket then go back and put sealer under the gasket before you put the trim back on. After all that, it may still leak…. sigh…