Low voltage at gauge, no charge?

The danger of the ammeter circuit is that the entire end to end circuit/ harness/ connections AND THE meter is marginal. They mostly worked OK with smaller alternators for the expected life of these cars.

The wire size was marginal, the bulkhead connector terminals were never designed for that kind of amperage, and the ammeter itself can loosen up and cause problems.

But now that they are antiques, now that people install big alternators, big stereos, etc etc, and now that the connector terminals have had half a century to oxidize, things get worse.

You need to repair or bypass the bulkhead connector terminals, not just the ammeter terminals.