Now you know the wire from tank to gage is conducting properly. You have narrowed the problem to two possibilities: first how well the rubber plug at end of wire at tank connects to sending unit’s terminal; and secondly is the sending unit grounded properly. If I were a betting man I would say that the sending unit is not grounded well enough, and is adding additional electrical resistance to this circuit. If total resistance of sending unit and ground path added together is greater than 73 ohms the gage reads empty. This also correlates with previous low fuel gage indication when full.
Metal fuel line is factory path to ground for this circuit; this path can become corrupted from corrosion at each of fuel line’s brackets, and tube’s corroded surface. Be sure to scrape fuel line’s surface to clean metal when bridging rubber coupling between sending unit and fuel line. You may have to run a new #16 gage ground strap from sending unit’s fuel tube to frame of car for a voltage drop free ground.