I love that crazy steering wheel. How long ago was the "custom" work done? Cool differential cover. Engine looks like a 216. The picture of the starter shows the foot pedal linkage. When you stomp down on it it pushes down on that button closing a switch and completing a circuit from the battery to the starter motor. So, yes the starter will turn with the key off. And usually a 12 volt battery will not hurt a 6 volt starter, if it's not cranked too long. It's common to keep the old starter when upgrading to 12 volts. The red wires connected where the battery cable connects eventually goes under the dash to the ammeter and the ignition switch. The ignition coil should not have any power with the key off. If you have a 6 volt coil a 12 volt battery will ruin it. I'm guessing there was not a battery in it when you started? Maybe you can take the coil to a parts dealer and they can tell you. Or look at the taillight bulbs. They probably won't be marked 6 or 12 volts, but you can cross check the numbers on them. The power inverter may have been for 6 volt accessories, like heater blower, radio etc..? Good luck.
