Epoxy does not break down when fuel is splashed on it, or when hot exhaust residue hits it. And the oily mess won't usually affect it over a few months.

But put epoxy under fuel or where it's exposed to it for long periods of time and it will break down. And J&B is basically a metal filled epoxy.

If you don't expect to use the fuel tank for very long, the J&B should work for awhile.

I made some fuel line plugs by dipping the ends of 2-56 bolts in J&B. The J&B was basically to give a bulge. They worked for about 6-8 months and I started getting crap in the fuel. And one day, noticed when I pulled the plug the end stayed in the fuel line. And when I picked it out with a pin, it came out is bits and pieces.