Are you sure it's not the sending unit gasket or filler tube seal?
Aftermarket sending unit lock rings are notorious for being incorrect (too thin and flimsy) and leaking all over the place.
Until we pull the tank, can't say where its coming from. The sending unit was new, and it didn't leak the first couple of fill ups.
I decided the roll the dice and purchase a new old stock tank for a 64 - 67 Valiant 200. If it doesn't work, I can return it. Alternatively, there is a fabrication ship in El Paso but the cost will be quite heavy, which I will do, as a last resort.
The fellow at the fabrication shop did say the sealants routinely fail as the ethanol dissolves it, so he says. He says that ethanol also plays Hell on aluminum, as on the atomic level, aluminum is porous, which is why it is so light.