The adjustment at the dizzy is for the home mechanic/farmer to advance or retard the timing based on the octane rating of the fuel he purchased. Back in the 30s and into the 50s, good quality gasoline wasn't always available so a farmer out in the middle of nowhere might be stuck with some sort of 80 octane garbage that would require a quick adjustment to retard the timing.
Turning over slowly is either a bad battery, poor battery connections, inadequate gauge wires, poor ground connections, or a failing starter.
If the engine spins over at normal speed and still doesn't start, I'd look at the ignition system.
Clean all cable connections. Make sure your ground cables and their connections are all up to snuff, have your battery load tested at Advance Auto or whatever. If all of that checks out correct, then I'd say your starter probably needs to be refurbished.
I've never heard of a polarity reversing switch.
I assume everything is 6 volt.
