High voltage Dart -67
Over 16 would SUGGEST (but not verify) that this is more than the usual voltage drop in the harness problem
The FIRST thing we need to know is what it is that you are running for an alternator/ regulator, AKA is this isolated field with the 70/ later Mopar regulator, or something else?
Is the regulator external or integrated in the alternator. PHOTOS may help, as it may not even be 'Murican, AKA Bosch or something.
IF your ignition switch does not have voltage drop across the switch itself, it can be made to work, and depending in the ignition system setup (I guess you said ready to run) but does it use a ballast resistor? Anyhow the switch can be made to work, but you may need a / some relays to relieve strain on the switch, etc
The BASICS of ANY alternator VR setup are this:
1...(Nowadays) you must have a good useable and COMPATIBLE battery. Some battery configurations like AGM etc / others like a little different "float" voltage but 16V is way out of the park. But the battery must not be sulfated, etc
2...Alternator and regulator must be compatible, AKA isolated field, grounded field, whatever, and of course with NO FAULTS in the field circuit
3....Regulator MUST be grounded to the extent that you have near ZERO volts between the regulator ground point (mounting, body) and the battery NEG post
4...Regulator power source and or sensing wire MUST have near zero volts difference between that point and battery POS post.
The TRICK is that voltage testing is often geared towards actual operating conditions AND you are testing for voltage drop in a circuit which is affected by current draw. THIS MEANS that you cannot willy nilly disconnect part of the circuit during testing because you will CHANGE these conditions, current, and therefore the drop
Again (photos are good) we need to know specifically what alternator and regulator you are running, to start with, and year model of the car will help as well