1975 Dart Sport - Alternator Issue?

The way to test your charging system is to have everything installed on the car and run a load test. Most good shops should have a carbon pile load tester. It may be that the alternator is working just fine and the regulator isn't. Simple fix if that's it, but, you won't know for sure without a full system test. Your year has notorious wiring problems, mostly at the bulkhead connector. Any corrosion in that connector can cause all sorts of electrical issues. Depending on where some of your add-on accessories were tapped in, you may be overloading one or more of those connections. Only install an alternator with the proper rating for your wiring harness. All the ratings up to 60 amp use the standard harness. Higher output ones such as the 100 amp you mentioned need a heavier duty harness, different ammeter, and a different voltage regulator.