The sending unit grounds to the metal fuel line. The metal fuel line grounds to the body with the metal fuel line mounting clips. Plenty of areas for grounding failure because of corrosion after 50 years. There is also a clip that jumpers over the rubber fuel line to connect the sender unit to the metal chassis fuel line. If you dont have this jumper clip installed, then you have no ground from the sending unit. Make sure you have this installed and a good ground from the sending unit body through this clip and fuel line to ground to test everything before pulling just the sending unit and ruling that as the cause.
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