Magnum truck alternator

Measure those two suspected "field terminals" with an ohmmeter. If a reasonable resistance between them (say 20 to 200 ohm) and they are isolated from the case and the output terminal, I would say they are the same isolated field terminals as in a square-back alternator, so should work the same with an electronic Vreg (triangle connector). Polarity doesn't matter. I would expect the engine controller (PCM) controls the alternator in factory Magnum motors (typical in new cars), but you don't require that.