I've never tried running the engine and checking with a multimeter but to me that sounds like it would not work
Try it with an analog voltmeter- - - -the kind we used back when these trucks were new. There was no such thing as a digital multimeter back when I learned automotive electricity. I can also check point dwell with an analog voltmeter, since it reads an average of the points open/points closed voltage which can be translated into a percentage of "points closed" time otherwise known as "dwell". Us oldtimers aren't really as dumb as everyone seems to believe.
Jerry