These transmissions are Borg Warner units and were used by many car brands. There are 5 components that go along with the transmission itself; the relay (usually on the firewall engine bay side); the solenoid (on the tranny); the governor (also on the tranny); the Overdrive Bowden cable (under the dash); and the kickdown switch (under the gas pedal).
There are many, many ways to do a 12 volt conversion. You must have a 12 volt solenoid and I suggest a 12 volt relay. The solenoid draws current in two ways (one to engage the pawl and one to hold the pawl) so you cannot just “step down” the voltage. Solenoids are interchangeable among car brands for the most part so long as the voltage is correct. One would just orient the solenoid shaft to the transmission by a simple procedure. Some solenoid brands (like Delco Remy), however, would be only for a specific transmission.
The absolutely best vendor I have found for Overdrive parts and knowledge is http://www.fifthaveinternetgarage.com/
. They even have new solenoids available. I would start there.
Google will give you lots of hits and instructions as well.
Here is one very good write up: http://www.fordification.com/tech/overdrive.htm
And another resource: http://www.tocmp.com/manuals/trans/Borg ... Overdrive/
I do have a 12 volt relay if you need one of those.