Sending Unit has me stumped!

[Ryanz]

I have a 1971 that we restored a year or 2 ago. New sending unit and gauge cluster. Everything worked until the end of last summer the gas gauge quit on me. After reading up I determined it was likely the brass float, so I removed and replaced with plastic float as it appeared to be taking on fuel. I've put it back in but must have a ground issue because it's reading over full (3 O'clock).

When I hook it up outside the tank, ground it manually, and move the float and it moves the gauge accordingly. But when I put it in the tank and hook it up - it stays at the over full position. Even when I try to run a ground to it I can't make it work properly. I've read all of the articles and I believe my ohms are correct. As I move it around inside the tank with it hooked up to a ground wire I can get it to work sometimes, but the second I secure it it pins over full. I even bought a new sending unit but have the exact same issue. Any suggestions would be welcome.