Alternator wiring

You undoubtedly won't get much of a reading with an external shunt such as you describe. I have no idea what the meter movement is in the later Mopars, Ferds and GM external shunt meters, but they are MUCH more sensitive than the original Mopar "full current" meter

You might take that approach---adapt a different meter movement as some of us did the voltmeter conversion, AKA same idea, and go from there. But the factory "external shunt" stuff all of them simply used the harness itself for the shunt, and you can imagine, not very "calibrated."

In the case of Ford Rangers and the mid 70's Ferd pickups, example, I'm familiar with, these were pretty "numb" generally. That is, you get it and crank on the headlamps, and you have to REALLY look at the meter to see if it actually MOVED.