1965 dart dash voltage

Nichrome winding in your gauges is approx' 13 inches of approx' 1.5 ohms per inch at 68 degrees. I've seen total resistance vary from 19.8 to 20.8
There is enough adjustment in the design to compensate for this variance.
If that mechanical limiter lost its ground it simply went off. In fact, lost contact at that slither of metal on the back of the gauge has been the only fault found in several cases.
The little 7805 regulator has no fault protection. If it looses its chassis ground it will pass full system voltage right through from in to out and fry these gauges in as little as 5 minutes. The factories 12 volt supply is can be 15 + volts after some battery drain.
You wouldn't properly heat sink the 7805 to a round surface inside or outside the can.
To put the limiter inside any gauge was quite foolish in the first place. And you consider following that? Just buy the RTE unit. Mount it where you can you access it if ever necessary.
The winding in your fuel gauge looks very very good for its age and believe me I've seen a damn bunch of them. Majority look more like the winding on your limiter. Or worse the Bi-metal beam bowed at room temperature too. That's the underlying reason RTE suggests we open these gauges. If the gauge is toast, their regulator alone will not fix it.
Don't do something foolish. You came here asking for guidance. So follow it. Happy moparing straight ahead.