single field to dual isolated field alt

I suggest a test. Drive around for a while with the alternator disabled and see if the cluster Vreg still dies. To disable it, just unplug the field connector. Recharge the battery each night. If not using the headlights, it should run all day on the battery.

The charging circuit is fairly simple and robust, so unlikely the Pertronix III could affect it, but never say never with solid-state electronics. My guess would be a bad sense voltage at the alternator, due to voltage drops in the bulkhead connector and wiring, as 273 suggested.