Gas gauge woes..

The thing is the battery is (or at least should be) at body ground

The TANK and the sender may not be. You are talking about a tank, subject to rust, with a sender, the compression ring, sender, and tank flange all subject to rust, and even the fuel tube--which was what the sender was originally jumpered to with the little clip, is "not" really grounded to the body!!!!

So when I say check sender tube to body ground, you are checking whether the sender is actually grounded.