Cranks but won't start. Unplug ground at field spade on Alternator and it starts?
Not really. The starting voltage is worth checking so you have that as a reference. But a weak battery then should show major charging on the ammeter when the alternator is output is available at 14 Volts.
Yes some (alot?) of the replacement alternators draw a lot of field current but I've not had issues starting when using them. And the ballast resistor should further limit the field current during start.
One thought is to work back from the alternator.
So next test is starting with the field lead disconnected, at the field terminal on the back of the alternator. Effect should be the same as disconnecting the grounded brush from ground.
ANd it shoudl also be the same as disconnecting the field lead at the regulator.
The last thing in that circuit you could disconnected and still have the engine run after starting is the ign side of the regulator.
You almost did this
I thought what if I unplug the Ignition 1 wire at voltage regulator and car fires up immediately?
That will verify the problem relates to the field circuit. So do it. Then go to the other side. If its in the alternator, I can't quite picture what the isssue is, but you ]'ll have narrowed it down to that. Then its your call whetehr to return or open it up.