Leaking gas tank question

So the weather has been good. I drove to my mechanic's. Eventually, with the car on a hoist, we found a pinhole at the bottom of the tank. New tanks, made by Spectra , in Canada, were to my surprise, readily available. How many of these things can they sell? When the old tank was lowered, I looked inside. I couldn't believe the sludge in the bottom. Disgusting, But I guess its 46 years old. I replaced the sending unit, but used the original lock ring. Thanks to everyone. My Dart is back in the garage for the winter.