voltage problem 74 duster

12.5 volts means the system is not charging at all. The nominal voltage on a fully charged 12 volt battery is 12.8 volts.

If it is a single field wire alternator and it's a 74 car you will likely need to switch back to the old style voltage regulator. The wire from the regulator to the alternator in the 74 style system switchs the field winding to ground. Sounds like the Mitsu alternator needs needs the regulator to apply voltage.