Gasoline Proof Epoxy???

Well, you can always take the bull by the horns ! Mix up two blobs of jb weld, let it cure for several days at room temp, then put one blob in a closed jar of gas for a few days and leave it to see what happens. Compare it to the unexposed blob and see if it changed somehow, either it will soften or dissolve if it cant hack it.

You could also contact an outfit that does head work and ask them -

Speaking from experience - I have an old 4 bbl 273 intake that I resurrected. The thermostat flange had been corroded so bad that someone put JB weld on it to fill in the pits. I soaked that sucker in solvent, and acidic and then basic water solutions, nothing would touch that stuff. I ended up milling the flange to remove the crap - and clean up the surface again. Its pretty tough stuff.