I will add you are safe, more or less, from accidentally crossing wires and ruining things when mixing differing wiring harnesses. Even though instrument clusters and engines can vary, the pins (even wire colors) used in the bulkhead connectors over the years (1970-1974) are the same, more or less.
The rallye dash calls for a different oil pressure sending unit on the engine, than the standard cluster. This may explain why the oil pressure gauge isn't working properly. The correct sending unit is as fat as a golf ball. While the standard one is much smaller.
Also, the rallye dash includes an extra wire from the engine side to connect with the tachometer. This may explain why the tachometer isn't working, if it's missing.
The (wiring diagram in post #2 shows the) voltage limiter you refer to impacts the fuel, oil pressure and temperature gauges. If any of those gauges are working properly, your limiter is likely in good order.
Troubleshooting electrical takes patience and time. It isn't hard, but it CAN be mysterious and confounding. Your best tool may be a digital multi-meter (DMM) and if you don't have one, grab a cheap one and learn to use it.
If you have deep pockets, you can pay another to do all this work for you. But even they, may need to systematically go through things, test things, check connections, etc, and bill you for their time.
If you have patience, or you don't have the funds to pay a mechanic (at $100+/hour) you may learn a great deal more about your car, by trying to work through the issues, yourself.
One really great thing about this kind of work, is the color of the wiring is nicely documented. And with the help of veterans on this forum, I am confident you can figure this out.
