Fuel Tank Locker

I am trying to replace the fuel pump on my 99 chevy blazer LS 4x4 four door. My question is does anyone know what size the original locker ring is? I have ordered a part from advanced auto, summit racing and another local shop. The ring that i keep getting is not large enough for the tank. I am 99% sure that the tank is original, the old ring was rusted and chunks broke off when i removed it, and now you can see into the gas tank.

Doesn't the fuel pump sending unit cover the entire opening of the pump. The locking ring should fit over the sending unit. Once the sending unit is in place you should not be able to see into the fuel tank.