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

Try this. monitor coil + to ground. Turn key to run, and then disconnect the alternator field. Note voltage rise. Then reconnect, and disconnect the push on terminal at the voltage regulator. Should be same effect

Now move your meter to the opposite end of the ballast resistor and again note key in "run" voltage, and again disconnect alternator field and note drop if any.

This is getting to be a puzzle

IT also may be that the battery is just plain too weak. You need min. of 10.5V or higher at coil when cranking

Have you rigged a spark gap and looked at spark during this problem?

What you want is a METAL (copper/ steel) core wire to replace the coil secondary wire. This can even be low voltage wire if you "hang" it in the air during testing. "Rig" this to a test gap if you can get it out to 3/8-- 1/2" or so. When cranking, spark should be nice, not snappy BLUE and at least 3/8 to 1/2"