Temperature gauge does not work on my 1970 340 Duster ?

There is only 1 temp sensor listed. The only difference is aftermarket quality.
The most important factor is water temperature. You must confirm that the thermostat is working and the water is in fact hot. Of all the gauges I serviced over a 8 year period only 1 was returned. That owner claimed the gauge reads waaaay too high. He didn't want to believe his recently rebuilt engine was running too hot. I proved again the gauge to be correct and sent it back to him
Is yours the opposite condition?
A digital volts meter on the mechanical limiter shows a lot of changing numbers. 2.XX to 3.XX that should average 5.XX volts. Most of us choose to replace that antiquated mechanical limiter with a modern solid state regulator. Several to choose from ( ebay ) starting at about 30 USD. Those are universal in design, thus requiring some alternate mounting and connecting by the end user. The only one that is nearly exact replacement for this application is about 55 USD ( RTE ). Plugs right in like OEM.
Any shop ( myself included ) that would service/repair your gauge(s) would demand a solid state regulator be installed. That steady stable output voltage that can be confirmed with a digital meter means fewer questions and extended gauge life.