1975 Dart Sport - Alternator Issue?

I doubt "that particular bulb" is causing the issue. Try the same tests with the parking lamps and or headlights.

Very FIRST thing to check is the belt tension and check the belt and pulleys for slippage and glazing

With everything in the car you can think of, the heater, lights, etc, the alternator should be able to maintain a minumum of 13V at least after it has recovered from the starter

So start and warm it and monitor battery voltage. Run it at a good fast idle to simulate a cruise of at least 35mph. Then turn on various accessories and see if it keeps up

There are further tests don't go replacing parts just yet