All,
If the gauge goes to full tank when grounded, this is something else I experienced.
Some of the fuel sending units I have received work just fine, but are not “adjusted” properly to the tank. They are to far above the floor of the tank and when full do not register full because they are only moved half way through the range of measurement.
My brothers 64 Comet Caliente did this. With an empty tank with it still in the car, I was able to bend the arm to get the float to rest on or near the tank floor when in the empty position. In fact we played around to get it to show empty with 2 gallons in the tank so his son would not run out of gas. “Son, when it says empty you best be near a gas station!!”
Hopefully you have a drain plug or just bring it home empty. BE VERY CAREFULL around any gas sending unit. Us a brass punch to spin the ring. No sparks around the tank. Especially an empty tank. They are the easiest to light off. Old tanks have the most gas imbedded in the metal. A new tank can be blown out fairly well with an air compressor with the sending unit out and the gas cap opened. Just carefully blow some air through it. If you have an assistant, you can keep a gentle breeze through the tank when you work. I was a radiator repairman a few hundred years ago and also fixed leaky tanks. Always was a rush to put flame on a gas tank!!!!
Rob