I own an old, but good, '84 Ford Mustang. I've always noticed that the voltmeter on the dash stays at about 13 volts until I turn on the headlights. Then it dips way down to less than eleven volts.
It does almost the same thing when I turn on the rear defogger. The needle almost touches the red zone.
Analog automotive gauges have been notoriously inaccurate over the years. Analog gauges use a needle as opposed to a digital read out.
What you've described sounds like a somewhat normal response to voltage when adding significant "loads" on the system. A load is anything that uses some of the power being produced by the battery and/or alternator. The headlights and the rear defogger are a couple of big ones in addition to anything else that you may have had on at the time (brake lights, wipers, A/C etc).
You have probably noticed the drop on your voltmeter while standing still with the engine idling. You also should notice that the voltage will climb back up as you take off and the engine speed (RPM) increases. This tells us that the alternator is working.
When dealing with a car this old it's still a good idea to have the battery and charging system checked out by a professional with the proper test equipment. This will make sure that you're not driving with a weak alternator or battery, bad electrical connections or even something as simple as a loose belt.