Does the change to 12 volt affect the Ford Coils?
It will improve performance on battery. It will make no difference if you run on mag. 12 volts can, however, overheat a coil if it is left buzzing on 12 volts. It doesn't hurt to RUN on 12 volts, but if the car doesn't start immediately and you let it sit with one coil buzzing while you look for the problem, it will overheat that coil.
My "main driver" has used coils and 12 volts for about 7 years.
My roadster was already converted to 12V and I have had it about 2 years the family that owned it previously said it was converted in '95 had no problems.
I believe it was Mike Gossor (sic) that ran tests and a chart concerning coils & voltages. It actually showed a slightly better coil operation for 12 vdc over 6 vdc though it might not be noticeable in actual service.