If your engine starts easily without a LOT of prolonged cranking the 6V starter will handle 12V. That pretty much makes the 12V conversion just a change of light bulbs, coil, condenser, and a voltage reducer for the heater. Possibly the horn or a reducer here too. You can change the heater motor if you like. If the old wires are in decent shape they are fins as they are bigger than the 12v system wires. Mt 270 GMC engine is a '57 so it was 12v. So for a while I ran a 12v starter on 6v. It was OK for the first start of the day but once the engine was warm it didn't work. I have run lot of 6v starters on 12v both Ford and GM and never had one fail.

"I wonder if God created man because he was disappointed in the monkey?" Mark Twain