A guy I know welds Diesel heavy equipment fuel tanks all the time. He fills them full of fuel and then starts welding. He has been doing it this way for over 20yrs now. I would call him and ask about your tank but he is curently fighting for his life in the burn unit as the last tank he did (last week), did not want to be done.
I know how good it feels to do it all your self but some things are just not worth it. You can try a JB weld type of product but please don't try soldering/welding it up.
Kevin
LFD Inc.