Tank you very much: advice please

Tank you all, yes it is a 1960 A Body, as pictured in the Avitar to the left. Finding a NOS may be a trick- are the tanks interchangeable for a few decades, or is this a one off? I take it there is not particular way to seal the interior of the tank? The guy is somewhat of an icon among classic car enthusiasts, here in El Paso. He may be thorough to a fault, and wishes not to send me down the road with a gas tank ready to come apart like an old water heater, would be my best guess.