For several reasons i want to change electrical system in my 1935 Ford Coupe to 12 volt...
If your wiring is still original I'd just rip it out and put a new 12v universal harness in the car. Speedway has them for about $150.
If originality is of any concern to you at all, I would think twice about this solution. As a matter of fact, unless it is a full on "Hot Rod", I would carefully re-consider my original decision.
There is a reason Detroit and other car manufacturers switched to 12 volt many years ago. I switched my '48 30 years ago and never looked back. Wouldn't even consider 6 volts.