use fields and armature from an Auto Lite 57 Ford for 12V.

My '51 Hornet was converted to 12v when purchased. Went through many starter drives and ring gears, as the starter drive would slam into the ring gear. Even with a Jack Clifford modified 12v starter with the Ford parts. This was for a stick shift. Hydramatic ones have a shorter starter drive. Converted it back to 6v with no problems, starts right up and can use the original heater and radio.
If you want brighter headlights, they make new 6v halogen ones, and carry a spare in your trunk. Or you can get a 6v headlight relay, and the stock headlights will be much brighter.