voltage across battery when charging ?

[82 GT]

Now that you have a ground to the frame, is there one more alternator you can try?
That will tell for sure if the PA alternator is bad since you are now getting 13.3v with a different alt.
Sometimes an alternator with a bad diode will not always show up on a test bench.
I would try one more alternator, if you can, and if you get a good reading I would send the PA alternator back to them and let them test their own unit.
The guilty finfer seems to be pointing at the PA alternator. It sounds like you have it pretty much narrowed down to the problem!

[Unit 5302]

Just as an FYI, much below 12.3 volts, the battery probably won't have sufficient voltage to start the car.

12.4-12.6 volts is considered charged. 12.3 is pretty low, and 12.2 is discharged. Below 12.2 is pretty much dead and you shouldn't expect the engine to crank.

The only place I can think to check is at the union of the main power lines off the alternator. While two positive leads run off the alternator, they merge into one 10gauge wire, if I remember right (been a long time). That connection can get severely corroded which can definitely lead to low voltage coming from the alternator.

I apologize if this was already suggested, I didn't read the whole post.