I can't help you with a welder. But I can tell you what I did.
My tanks were not leaking, I pulled them out to install larger pickups and found I had some pitting on the bottom of them, thanks to Eliminator installing them in a bed of carpet. A couple spots I was able to push a pick tool through. I welded up the holes. The other areas of light pitting I cleaned and sealed with JB weld. I then cleaned and sealed the inside of the tanks with this cleaning/etching/epoxy kit (http://www.kbs-coatings.com/KBS-Gas-...ealer-C38.aspx). After the inside dried I sanded the outside of the tanks and applied west systems epoxy to them. I spent a little over $200. bucks in materials restoring the tanks.
I sealed my tanks about a month ago and just put gas in them tonight so I cant answer how long this sealant lasts or how it performs. The sealant that leaked onto the outside of tank was a biotch to scrape off with a razor blade and seems to have one hell of grip on to the tank. Only time will tell for sure.