The gage marked 'alt' does NOT read at a terminal.
It is an ammeter. It deflects to show how much current is flowing through its internal shunt.
It only deflects if current is flowing through it.
Unless someone has wired new loads onto the battery, the ammeter shows the flow rate the battery is discharging or charging.
When the battery is charged and the engine is running, the alternator should handle everything and the ammeter should show zero.

The system voltage needs to be checked at several places to be sure the regulation is in spec.
* Alternator output stud.
* Regulator ingition connection
* battery Pos.
Also check that there's no voltage drops in the ground. Voltage drops only occur when current is flowing though a resistance, so you'll need to turn the heater fan or lights on for this to be a helpoful check.

As far as the pertronix goes - assuming that's what it is. Its probably OK with the electromechanical VR, but you can check with Petronix. It's probably in the instructions if it can't handle the noise of a mechanical regulator.

With a replacement engine bay harness, you still will need to check the firewall connectors and also the headlight switch connector as described previously. Too many unknowns here to assume everything else is good.