Hello Yummy,
I hope I am not to late to suggest a compromise solution to your 6V - 12V problem.
Since starting is no issue for you, why don't you just switch to 6V NEGATIVE ground?
The advantage of that is that you can use ALL the vehicles electrics as they are right now and just use a 6..12V step up converter for you radio and a charging port for your phone (maybe even the original cigarette lighter). Now the vehicles Ground is still ground and you don't have to go through the trouble of isolating all 12Velectronics from the car's body.
I think I would o that if I would want to use 12V stuff (but I don't care).
Here is how it is done:
The only thing that needs reversing are your
battery (of course), your
voltage regulator (re-polarizing) and your
clock.
1) start with the clock. Remove it from the car and install it back once you are done with the other steps.
In the "Early Bird" Magazine was an article about repairing the clock, but is shows how to switch to negative ground:
Just move the small prong that is riveted to the housing from the + bolt to the - bolt.
(See first picture on the left - I added plus and minus and the arrow)
2) Now swap over battery cables - if they are too short, you might to experiment with new once. However, don't use 12V cable from the generic auto parts store, they don't have the required gauge for the current a 6V system needs when starting.
Don't start the car or switch ignition on.
3) Now re-polarize the voltage regulator
Use the left side description for Ford/Mercury cars.
4) put your clock back in.
Now everything should work and starter and engine will NOT turn backwards!
However, here is a caveat! I never did this to my car, so the experiment is on you to find out.
I just know that the car will still run, because an idiot at the dealer I bought mine didn't know about positive ground and charged the battery off-car and put it back in the wrong way. So i started it and it ran ok. However since he didn't re-polarized the voltage regulator it fried and it also fried the clock.
I did replace the voltage regulator, but found out later that I can just re-polarize its and it runs now nicely in my car.
I f you try that please let us know if it worked for you!
Have fun!
Bob