aftermarket volt/alt gauge wiring help

At the risk of complicating this, now would be a good time to verify if you have voltage drop in the ignition supply to the VR and ignition, and "do something" about it.

Voltage drop to your VR IGN terminal actually causes OVER charging. Also, if the voltmeter is hooked to something that is measuring "untrue" voltage, it won't give you a clear picture of what gives

Depending on what you have for a charging system (pre 70, 70 / later, or "something else") turn the key to "run" with the engine off. Put one meter probe on battery + and the other on an IGN "run" terminal, such as the IGN terminal of the (69 / earlier) regulator, or the blue field wire on a (70 / later) alternator. What you are hoping for is a very low reading, the lower the better. More than .3V means you have a problem, that is, 3/10 of one volt

One way around this is to use a relay to supply the ignition. "cut" the blue "run" wire, and use the lead coming from the key to fire the relay, then use the relay switched contact to feed the underhood "run" loads. You can fuse / breaker it off the start relay stud

THAT should give you solid battery voltage, and if you can run that back to the voltmeter, will ALSO give the voltmeter a true "picture" of what is going on.