I've never been able to seal a tank fitting with anything. I've tried JB-Weld, Epoxy & Liquid Steel but the gasoline always seems to dissolve or melt it away.
You really only have 2 choices, have it welded, that is if you can find someone who will do it. The tank will have to come out, flushed out and filled with water to prevent vapor explosion.
Buy a different tank. A new one will run you under 170 bucks at Auto Parts Fast at RockAuto
. Buy a used one at your local junkyard.