71 Dart overcharging

And after you do that (the above) make some voltage checks

I don't know how you have your cables wired, so you need to itemize, that is exactly what and where is the grounds, and what size, and how large is the POS and how large

This is LIKELY to be 'voltage drop' at the VR

With the VR powered, and engine off, measure voltage from the VR case to the VR IGN terminal. This voltage should be as close as possible to "same as battery" and if it's off by more than .2--.3V (3/10 of one volt) that is the problem

Same with ground. Get the thing running and charging "fast idle." Using an extension wire, hook a wire to the battery NEG post. Run this up front, and hook that wire to one probe of your multimeter. "Stab" the remaining probe into the VR mounting flange. Again, you want to see LESS than .2V, the less the better. Then repeat the test with whatever heavy loads you can turn on, heater, lights, fans, whatever.

NEARLY forgot.......disconnect the VR and make absolutely sure the VR actually has control. It should stop charging.