NAPA has a gas tank sealer that is two part epoxy, that works great, specifically designed for gas tanks. You have to work fast as it sets up in a hurry. I suggest getting two packages as it sets up so fast you don't really have time to check everything out. Make sure you have everything ready and the painted tank parts covered.
I cleaned out mine as best I could, leak was under fuel pump, where they always are. then applied the product both inside and outside the tank. 6 months no seep or leak, and I had a hole.
Last year I tried another epoxy and it gave out half way through the summer. I tell you for $14.95 this stuff is a miracle and way cheaper than welding.