I have a top loading fuel sending unit on a 49-53 tank with the rubber gasket and it works. The holes in the gasket are smaller in diameter than the screws, providing a good seal. Mine did not leak even with the extreme tank pressure caused by the stupid aftermarket non-vented gas caps.
If you can't get your tank to seal with the rubber gasket, you need to look closer at the surfaces which you are trying seal.

1952 5-window - return to "as built" condition | 1950 3100 with a 235 and a T-5 transmission