Ric, no idea if this could be true of a '72 sending unit, but had a '97 Safari that began to do a similar thing, would read full all the time and maybe drop to 3/4 or 7/8 full when empty. When the issue first developed, it seemed to act up intermittently, some days seemed to read the tank level accurately. As it got progressively worse, there were times when it would seem to give a broader range over a tankful and I would hope that it was coming back to life. But eventually it was only accurate when the tank was full. The gauge needle would move, but the sender just had stopped sending a proper signal.
Drove it that way using the trip odometer as the fuel gauge.
In that van, the sender was sold as an assembly with the in tank fuel pump. Was content to live without a fuel gauge rather than pay the price for a new pump. When the fuel pump eventually died, replaced it and had a working fuel gauge for the first time in years (and only ran out once!).
The way I understood it, the sender operated with a rheostat and pointer. Depending on where the pointer was along the rheostat determined the resistance and therefore the voltage sensed and sent to the gauge.
I seem to recall it was the pointer (rather than the rheostat coil) that would wear so that it no longer generated the electrical signal it needed to.
Not sure I'm describing the idea accurately but the net result was that there was never anything wrong with my gauge or my ground or the voltage to the sender. Only way to fix it was to replace the pointer and coil as part of the fuel pump assembly.
If the '72 sender operates off the same principle (sans the integral fuel pump) could be the issue you are seeing.
Seems you could prove the circuit is good by plugging in to a known working sender and run the gauge from empty to full with the ignition switched to on (not running).
Hope this helps in some way.
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