Starting/Charging issues (long)

The two first things I would do in your case are to check the wiring at the bulkhead connector, and bypass the Ammeter. I had more electrical gremlins with my 66 than I care to remember until I did those two things.
For a quick and dirty bypass on the Ammeter, unhook the battery, take the hot wire from the left (driver's) post of the ammeter and move it with the other wire on the right (passenger's) side post.
Pull apart the bulkhead connector on the outside of the firewall that has the large red wire, usually in the upper right (passenger's) corner, and see if it's burned or arced inside the connector. This is very common on these old Mopars, especially if the ammeter is causing problems.
Once these two variables are taken care of, then you can continue normal troubleshooting procedures if you still encounter problems. Good luck.