67 Dart-upgraded alternator

Yup, what RedFish says. Under normal conditions you might not have a problem, but accidentally leave the lights on for awhile then come back and start the car and the alternator's going to be pumpin' current your wiring wasn't specced to handle when it was new -- and science has not yet given us the wiring that improves with age. It will be worth your while to upgrade the charging circuit. As you can see from this thread and others, there are various ways to do it.