Those old Yales used a prestolite charging system which was very poor. It also had an external regulator attached to the rear of the alternater. At best the alt would put out maybe 13.5 volts (on a good day). The voltage reading at the battery should be at least 12.5 V with engine running.

Sounds like the alt/reg isn't charging correctly. I would suggest changing the charge system to a Delco self regulating alt. There is a special alternator housing to accomplish this. I've done the conversion serveral times with no regrets.