i noticed a while back, when i was at my ACE hardware, JB had a gas tank sealer as well as other formulas for specific instances where the regular JB wasn't appropriate.
however, if it were me, i would take the tank off, hose it out good with hot soapy water, then fill it with water to almost the hole location and silver solder it.
take the right precautions, and you won't go BOOM !