Alternator discharging, bad alternator?

Or, use a voltmeter and test the battery voltage when the car is warmed up, is at FAST idle, and with the battery well charged overnight. The battery should then read 13.7 to 14.3 volts.

That vintage of original alternator is likely a 35 amp one, and will not put out a lot of charge. Look up 'roundback' versus 'squareback' alternator here to see which you have, or post a pix of the back of the alternator.