Replacement Fuel Tank Question

I had the problem with leakage from the sending unit area when I installed my new tank. Used the old lock ring and it still leaked. I found the issue to be the stamped sending unit opening lip does not always sit flush with the sending unit mating surface. I took a fuel resistant o-ring 2 1/8" by 1/8" and installed it between the sending and the tank. Used a bit of permatex 2a black just to hold it in place on the outside of the two locating tangs of the sending unit. Installed the lock ring without the outside gasket and it is leak free.