I believe that one of the problems with new tanks is that the metal is a different grade or quality. I have a new tank that came with my '69, it leaked, the spot welded retainer area the the sender, gasket and lock ring were deformed. I could have sealed it with the pro seal, but really didn't understand, got a new stainless tank, had the same issue after about 6 months. So I fixed my problem with pro seal. Hats off to those of you that got it done with the parts available now.
Good luck!