69 Dart GTS 383 4 Speed Completely Original

if you replaced the regulator with a cheap offshore one and it charged for a day and then quit, that's the first place i would look.
They have contact points in them, and if you take the cover off, you can see if the points look bad.
I would have the alternator tested, clean up the wiring, and install a newer constant voltage regulator on it.
Nice car BTW, and a smokin' deal to boot!