12 vs 6 question thoughts.
While at time the 30, 40, and early 50's cars and truck were produced the 6V system was perfectly adequate. And in talking add on loads even then the 6v systems are able to provide.
I do remember working around my Grandads garage as a young kid, and seen a mocked up 6V winch on a truck. Of course at the time it looked to me the cable to the second parallel battery and then to the winch looked as big around as my wrist.
6v systems can power anything with large enough wiring, adequate generator size and enough battery current.
On the other hand the military found the voltage pressure of 24 volt made for more power with smaller wiring and more punch in starting in the harsh conditions.
But the key to manufacture change is wire size and loads. With the primary area of concern in late 50's lighting and safety. Candle power needed to increase for better night driving at increasing speeds and better lighting for rear to approaching cars. While one could increase wire size and current. MFR went to 12 to provide the boost.
A simple view is if your restoring a show truck or weekend only driver without plans to make freeway trip or heavy city driving leaving the period adequate 6 volt system is the best one can do.
But if your creating a daily driver that will see freeway or fast hiway, bright city night driving or even unknown farm roads. A choosing to provide the 12v allowing increase lighting ability should be a choice examined for benefits vs cost of change.
IF the battery has the 6 cap holes of the 12v. I would also look for part numbers on your regulator and your generator and coil to check for 12v versions versus 6v.
