mustang gas tank identification.

I have a 66 mustang 289. my fuel gauge does not match the tank size I believe it is a 16 gallon tank.
the gauge does not register correctly .I bought a new sending unit Scott Drake for a 66 mustang twice .
neither one fits.is there a id# on the tank i can check to see if it is the right tank?



Thanks.
Halloween.

I mean the locking ring will not fit over the new sending unit.