Prove me wrong by making it work for him, that tank is probably FUBAR anyways but I can appreciate his effort. Very unlikely you or anybody else is going to seal copper to steel with a handful of aluminum rivets in the mix which would melt well before the solder. And, of course, make it all fuel tight. The act of heating that tank will likely open more holes in the steel anyways. Maybe you can build him a ghetto tank out of a shampoo bottle.