I'd try the routes slowrev mentioned first.
But, if all else fails and only the bottom is rusted out, you can have the bad section cut out and a new sheet brazed in. Not optimal of course, but I have one going on over 15 years [actually that one may have been solder]; these days you might even find someone with a steady enough hand & a MIG instead.
About the same time, I remember reading an article on a compound you can slosh around a [empty] gas tank to fill pin holes and prevent future corrosion, then you dump it and let the rest cure in the tank. Maybe it works, but didn't appeal to me.
Good Luck!