[Blas]

The ground I mentioned is the front loom ground, which includes the ground wire on the voltage regulators case

Regarding a higher output alternator, while many owners have gone to 100-120AMP alternators, your cars wiring design is really not designed for that increased level of power. And the AMP meter is only rated for about 50AMPS too.

A properly performing alternator/charging systen will only 'produce' the amount of current it 'needs' to feed the electrical needs users requesting power, plus a little more to keep the battery fully charged.

FANS = ~6 - 25 AMPS
ELECTRIC FUEL PUMP = ~3 - 6 AMPS
INSTRUMENT PANEL (7 x 0.27AMPS) = ~2 AMPS
BRAKE LIGHTS = ~3 - 6 AMPS
RUNNING LIGHTS = ~3 - 10 AMPS
MSD 6 OR 7 SERIES IGNITION CONSUMES APPROXIMATELY 1 AMPS/ PER THOUSAND RPM.
LICENSE PLATE BULB (2 TOTAL) = ~0.33 AMPS
F/R TURN SIGNAL BULBS (4 TOTAL BUT ONLY 2 USED AT ONE TIME) = ~4.2 AMPS

Keep in mind that you have a very early chassis too, so your wiring is approaching 20 years old.
I don’t remember all the details of what you have done to fix the problem so far.

Are you using the same alternator that was on the car for all the years is was a museum piece? And did the car just "sit" for years or was it driven occasionally?

I assume the battery in new?

Get a short piece of brown 12ga wire, put two alligator clips on the ends, clip one end to the case of the voltage regulator and the other to the mounting bolt for the alternator, any difference in the movement of the AMP meters needle?

An AMP meter in a properly operating system will jump when headlights, turn signals or fans initially turn on, but should settle down and steadily register approximatley 1 to 1.5AMP more than the total AMP draw being used by the cars electrical system.

There is a red indicator light just above you ignition key, when does it light up on your car? Is it on when you turn the ignition key to the 1st position? Does it ever glow a little bit when the car is running?