Not charging with headlights on

Were you reading the voltage at the battery, or at the back of the alternator?

I'd be curious as to what you are getting at the back of the alternator. The factory charging system on these cars is pretty finicky. If you are still getting less than 13.8V (it should really be more than that) at the back of the alternator, I would at first check to make sure the regulator has a good ground and that all the charge wires and connections are undamaged.

I had many issues over the years with the charging system on my own 70 Duster. I eventually got tired of it and swapped it to a Delco 12SI alternator and have never looked back. The thing works fantastic. Now, in order to do that, you'd have to do a lot of supporting mods...the MAD electrical bypass, big charging wire, and some custom brackets and a small amount of wiring.