I worked in a shop with an old time Chevy mechanic. If I remember correctly, he like using acid core solder for radiator necks. I would be willing to guess that it would work as equally well on a fuel tank. If you go the plumbers solder route, a HVAC technician showed me once that he likes brazing a joint with Silphos and then going over it with plumbers solder. -Just a suggestion.
