He may be correct as many fords used a resistor for their gauges well into the 70's! You can use gauges from a 70's pickup and insert them straight into a 48 f-1 !

If he's using a ford gauge and a ford wire and a ford sending unit, it should (in theory) WORK! Unless there is an optional gauge cluster in the center of the dash or some other wires have been "re-purposed?!?!?

Maybe try a separate (new) piece of wire to run from the gauge to the sending unit and eliminate all the other stuff as a test? I never heard from him if he did test the wire by grounding at the block the connection that would go to the sending unit and what those results were.. or did I miss that?