squareback alternator wiring in pre 70 car issue

One other thing to consider it the later alternators put out much higher amperage then the original and the ammeter can't handle that for long before it fails and leaves you stranded .

Run a 10 gauge wire from the alternator to the starter or battery to carry the load.
By pass the ammeter by moving one lead to the other post so both wires are on the same post. Install a volt meter.

Of course check for voltage drop from the battery + to the + terminal of the voltage regulator. This will test the bulkhead , ignition switch and ammeter bypass connections.