1972 dart alternator pulley shaft question

You might want to check both at the alternator output stud and at the battery to see if you have voltage drop. Also, what kind of load is on the system, IE do you have big pump, water pump/ electric fans etc, or the alternator may have a problem and is not outputting rated current.

Possibilities:
Battery still low or has a problem
Alternator has a problem and only partial output, or just is a small unit
Voltage drop in the charge wire
Excessive system load, AC/ lights/ pumps/ fans/ etc