Gas gauge problems

There IS NO frame/ body in Mopars, they are a welded unibody. You are simply making a poor connection. If the gauge thuds to full when the wire is grounded, either you.......

have a broken connector at the end of the wire and it is intermittent

or the sender is bad

or the tank/ sender is not grounded. Hook up the sender "normal" Connect a good jumper clip to the fuel outlet nipple, and make DAMN sure the other end is grounded. If the gauge works you have found it. If not, replace the sender

"What I use" for this kind of testing is a scrap of wire I can run all the way to "up front" and clip it to either battery NEG or something "bare" on the engine, like the carb, or vacuum advance can, etc