I don't have a lot to add but it would be very helpful to know where this voltage measurement is being made.
The ammeter measures flow in or out of the battery. The range on a '69 ammeter is 40 to 40 amps, with zero in the center. So pegged is over 40 amps and will overheat any and every weak connection.
Keep this in mind too. That's 40 amps to the battery. The alternator is also supplying power to its field circuit and the ignition. So the output wire is carrying at least 5 amps more than the current going to the battery.
