69 Dart GTS 383 4 Speed Completely Original

The main issue with Mopar wiring is the bulkhead connector. From sitting that long, the connections are probably corroded and not making good contact. This is a dangerous situation because this could cause a fire. Pull the bulkhead connector apart and clean/inspect the connections. Once it's clean, reassemble with dielectric grease on all the terminal ends.

Also, make sure all your grounds are good. New voltage regulators are available that look stock but contain modern electronics inside. I would look for one of these.