Low output at idle w/ac on?

Some ting wong. I do not believe those cars had the old style full current ammeter, and it SHOULD have a square back. Either there is something wrong with the charging system, AKA wiring, ?? or somehow a rebuilt ended up on the car that is WAY below amperage, That is, maybe you really have a 35-40 alternator thanks to some cheap rebuilder.

I would get into the manual and run a field test. Turn "stuff" on to cause a load, and then monitor battery voltage vs the output stud of the alternator, which will tell you some about voltage drop from the alternator to the battery.

Also with a bunch of accessories on, jumper 12V to the field and see if the low RPM voltage keeps up. Be careful with this and monitor battery voltage while doing so. Don't (try not to) let it climb above 15.5-16 or so.