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