Instant loss of charging... car is now dead on road...

If you have a low resistance connection from the alt out to the battery, you can't read 28 V at the alternator. If you manually apply "full field" 12 V across the alt field terminals, it should read <15 V at the battery. If over-charging the battery, it will start breaking the water into H2 & O2 but the voltage won't go up.

Verify you have water to the top of all the cells, and the battery reads ~12.6 V with one lead disconnected.

Measure the resistance from "Alt out" to the "Batt +". Should be <1 ohm (subtract the reading w/ multimeter leads shorted together). You probably will measure an open circuit because your ammeter or bulkhead connector is a broken connection.

There are constant warnings on FABO to check, clean, or bypass the bulkhead connector. Many run separate wires around the connector. It looks like the factory did that in 64, based on mine. In 65, they used special thru lugs w/ clamps. Maybe 66+ they got the bright idea to run the high ammeter current thru the regular 57 terminals.

Regardless, your simple fix now is to run a 8 awg or larger wire direct from "Alt out" to "Batt+". Your ammeter will no longer work, but your battery will charge, if the alternator field wires to the Vreg are correct. Anybody more motivated can look up my post on using diodes to bypass my ammeter at high currents to protect it, but a higher threshold and don't blame me if things melt.