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

I would pull that alternator off where you can SEE it and do some checking. Maybe the so called insulated terminal is not. Maybe the insulator is broke, or it is (should not be) grounded, or maybe the field windings are shorted/ partially shorted.

It may be that voltage drop in some splice/ connection or even the bulkhead connector terminals where IGN1 passes through are corroded and the alternator (if it is the problem) is causing a big drop

None of this makes sense.

Is this breaker points? With the points accidently open, key on, coil + will read same as battery because there is no coil current

With points "accidently" closed, key in run, coil + should read (varies) 5 ish to 10 ish volts. The VR/ alternator field circuit comes off the same "run" buss (IGN1) so if you are getting proper coil voltage in "run" then not likely the alternator is AFU.

Cranking, coil plus should be close to "same as battery." that is a simple circuit, one wire. It goes from ign2--usually brown--through the bulkhead--to the coil + side of the ballast, and it is what gives the coil "full battery" when starting.

You may be getting fooled by bad connections somewhere, AKA as mentioned, the bulhead connector, etc