The aftermarket senders are a piece of shit. I had the exact same issues he is describing. I had it in and out a number of times and every time I tested it out of the tank it worked perfect. Put it back in and fill the tank and it would not move from Full until the thank had a 1/4 tank and finally it just stuck on full. I had a original 3/8" sender I restored and it works perfect.I think you have to bend the arm in the tank down a bit Carmine, hook the gauge wires up while the sending unit is out and when you move the arm, you can see what the gauge is showing. Maybe run some long wires from the back to where you can see the gauge, or get a helper.
Don