Cranks but won't start. Unplug ground at field spade on Alternator and it starts?

Mattax said

Probably within spec. Slip ring to slip ring the rotor resistance should be 3 to 4 ohms at room temperature. The carbon brushes work well as a conductor when spinning but can show a lot of resistance when measured with a meter like that. But at least we know it's not open.

Good. So no short there.

Good.

Assuming the old alternator has a grounded brush holder, that would eliminate something related to the grounding wire used with the insulated brushes.

And in a previous test it started when the grounding wire was disconnected.

So that seems to narrow it down to the section of the circuit. Maybe there's a short or partial short somewhere in there.
Next places to disconnect during starting are each end of the field wire (R3), at the VR, and then at the alternator.

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I added my Unilite wiring to the diagram you posted up. That distributor was installed around 2008, worked flawless until 3 weeks back at which time the module went into a slow death mode where engine stumbled/missed like a low fuel supply issue, ran fine when cold, would die when engine warmed up, and eventually car would start and immediately die. Also found electronic voltage regulator that had been on there 20 years or so also stopped working. This was several weeks back and why a new ignition module, new alternator, new voltage regulator, new ignition switch etc was installed. Usually when Unilites burn up there is an underlying issue causing it so I been doing voltage drop tests and ohming things out during the parts replacement. Car ran fantastic for two weeks then when I stopped to get gas last Friday the no start issue surfaced without warning.

Prior to the electrical issues that started weeks back the alternator, voltage regulator, unilite module, and M&H engine harness had been on there since 2008 and all worked perfectly. I had car apart last year and a half doing full floor replace and 6 speed swap engine kframe taken out ect. I did all that work myself. Car ran good for a couple months once put back together.....