6V motors running on 12V will last quite a long time - BUT its not recommended and they will run very fast. I have done it and didn't like it, so I changed to the proper motor.
I would suggest:
1) a method to reduce the voltage to 6V for each motor
2) using an aftermarket set up like street rodders use
3) compare the existing physical size/mounting of the 6V motors and gearboxes to other 56 Ford products. They might be the same as used in Mercury or Thunderbird which are readily available.