how to bench test a 70 Dart instrument cluster?

Easy. Identify the pin supplying 12V from the key (and you likely have, and apply battery power to it.

The cluster is "poorly" grounded, OEM, through mounting screws. Put a wire pigtal under one of the common ground screws, and screw it to your dash frame or column support, when installing dash. Use that pigtail, of course, for the test ground.

With no power, carefully inspect and check resistance from the PC board pins to the IVR contacts. Check carefully from the board traces to the actual contacts. These fail, and it is wise to solder jumper bridges across from the traces to the IVR contacts. Also work the IVR in/ out a few times to "scrub" the contacts

The gauge studs can lose contact with the "eyelet" contacts on the PC board. Work the nuts loose/ tight/ loose/ tight a few times to scrub them. Consider replacing the nuts with "real" nuts

Rig appropriate test resistances from the sensor terminals to ground, and see if the gauges follow. They are same resistance regardless, of fuel, temp, or oil, if equipped

Check the harness connector pins carefully on the board for loose. You can clean them and solder them to the board.