Gas tank leak

Hey everyone,
Well my sons 68' dart recently leaked gasoline.......question is why? Well we looked for the source and found there are not holes and are no rust spots so no help there, but we found it was instead coming from the sending unit area. From what it seems the gasket either was not on good or not on in the right area (it was not centered but instead more upwards and the leak was where the lesser of the gasket was) or it wasnt on tight enough. So since he has pulled off the sending unit to check for holes there, and came up empty. This leads us to believe that the gasket location wasnt a sufficient seal and led to a leak. I've decided to make a bigger gasket myself hence we live in hawaii and no auto store carries the lock ring and gasket. My question is is there something im missing and what materials can i use to make a gasoline proof gasket? I was thinking quark. Opinions and some help would be awesome right about now. THANKS.