how to bench test a 70 Dart instrument cluster?

Varying test resistances.......this has been posted many times.......

This photo came from an aftermarket "copy" of the OEM gauge test box, which is simply 3 resistances. They check if the gauges go to "1st mark," middle of scale, and "full" mark

So you need something that is either these three resistors or a variable (rheostat) such as a spare fuel tank sender, which you can set with your ohmeter

The three resistors are in YELLOW

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Now what you're functionally testing is the complete circuit path which is as follows in your test set up:

From POS post of battery---to ignition "run" pin of PC board harness connector----to board trace to IVR unit---via board traces to the oil and fuel gauges---through the gauge---out on the board trace and to the PC board connector pin for the oil or the fuel sender---connect your resistor----to ground........and back to battery negative

The current through the resistor--more current for a lower resistor, causes the gauge to read HIGHER on the scale.