Make sure you have all good, clean, solid grounds from the body to engine, and engine to battery.
A slight fluctuation in volts and amps were common to those cars at idle.
The most important thing is to see at least 13.5 volts at 750 rpm with no electrical accessories on, or no less than 13.2 volts with h/lamps and heater on.
Above 1500 rpm anything more than 14.5 volts maximum is an overcharge condition, and has to be repaired or you will cook the battery