6 volt to 12 volt conversion question

Actually all 6V were NOT positive ground, nor were all 12V neg. ground. I can never remember, but for a few years Chev and GMC pickups (6V) were OPPOSITE polarity ground.

And there has been some "heavy trucks" that were 12V pos ground. These used to be a real PITA for two-way radio shops, as some trucks had communications radios. Earlier (trunk mount) two way radios were often selectable for ground, but more modern transistorized radios required some sort of inverter. Expensive.

With that out of the way, MOST electric motors don't care as to polarity. Wipers, heaters, starters, as long as they are the old common "universal" motor that is "wound" field instead of more modern permanent magnet (mini starter) they will be fine, either polarity. IN FACT THEY WOULD operate on AC!!!! (Old 120V electric drills, vacuum cleaners, etc are essentially 120V versions of your heater motor. They would operate on 120V DC, either polarity and of course AC. This is why they were called "universal" motors.

So, what do you need?

Starter will be fine, and solenoid "might." If you can change the solenoid, I would

You will need 12V coil and matching resistor, or your choice of modern ignition

You will need "Runtz" instrument regulators for each electric gauge, such as fuel, temp, oil pres, etc, or build your own regulators, which you can do

If the ammeter is in good condition, you could use it. You will have to reverse the wire connections on the back for pos to neg.

You will need dropping resistors for wiper motors and heater blower, or change the motor.

Obviously change all the lamps

Obviously a 12V gen/ regulator or 12V alternator