Battery Measures 15.5 vdc

Here's a diagram of the charging circuits for '68 Barracuda Valiant. Dart will be similar but you may find small differences.
The Voltage Regulator (blue Ign wire) determines whether to pass current to the field based on the voltage in the blue wire from the ignition.
It should be the same as at the Alternator output (BAT stud) and the Battery's positive.
If not, look for the voltage drop.
If it is, check for poor grounds.

tips:
Field is shorthand for electromagnetic field. The windings inside the alternator create an electromagnetic field. When they are spun, the magnetic field induced electric flow in the non-moving windings. That's the power the alternator can provide.
Also notice power from the alternator does not go through the ammeter except for charging the battery. So this is what the ammeter tells us.

Charging-diagram68BV+Headlights-A.PNG