Here are a couple of electrical items that have worked well for me. I use the voltage monitor in place of an assimilator and it has alerted me to electriclal problems more than once. It is bright enough visible behind a stock jewel in the headlight (amber works best) however dims to half brightness when you are cruising and all is well (steady green).
The other is a non polarized LED H4 bulb, it too works fine with my positive ground system.
The video shows the voltage monitor at "Bootup" and then in a 2 states with the headlight on or off. FWIW I have a '73 850 too.