Steve,
The anti sqeak is maybe an 1/8".
I measured mine. 5 3/4" from the floor on the inside the tub to the filler cut out. This may be a bit off. I did replace my floor pans. But again, original tub, a repro tank, anti squeak, and it all aligned just fine.
Could the tank be the problem? After I replaced my floor pans, there was no more sagging floor. All the sudden, the original tank filler neck didn't fit right. I had to cut the neck off and reposition it. Ultimately, this original tank leaked at the fuel line connection, so I replaced it.
Tim