It sounds like you need to find a competent starter rebuilder. Anyone who swaps field coils to the wrong ones doesn't know enough about starters to be tinkering with them. Why was the starter "rebuilt" to the point of replacing field coils in the first place? It would probably be best to swap to an un-bubba'ed starter and see what happens then.
John's new 261 rebuild was a bit reluctant to crank on 6 volts, especially with the Mickey Mouse cable setup we were trying to use. I swapped ignition coils for a 12V unit, got some real battery cables built from #0 welder cable, installed a hot 12V battery, and the thing started on the first try afterwards. After the cam break-in run we switched back to 6V and had no problems. I'd suggest you do something similar to get the engine running.
Jerry
