‘66 Barracuda Fuel Gauge Pegged!

But, when I put a probe into the mating connector, the one that goes up into the dash, I get a continuity reading of 35 ohms (battery is disconnected).
This tells me there’s a short in the upper harness, correct?
No sir, the fuel gauge is 20 ohms. That may have a backward path to ground through a limiter or regulator.
I dont know what you used for a 5 volt power supply at the workbench or what you are using in the car. Far too much power going into a gauge will peg it and look like a short circuit.
If you have RTE regulator, did you disable or isolate the original mechanical limiter?