74 Dart Wiring diagram

This can be one or more of several problems. The alternator output "path" is:

Alternator output..........charging wire........TO SUSPECT WHITE UNDERHOOD CONNECTOR......to bulkhead connector........BLACK to ammeter.........out ammeter on RED.......out through bulkhead connector.........fuse link.........starter relay "big stud"........and to battery

ANY of thoses points there including the wire ends at the alternator, any of the connectors and the wire ends at the ammeter AND the ammeter itself can be to blame.