This may be a bit more complicated than it first appears. Here is a schematic of what the wiring looks like (page 7-20). http://www.studebaker-info.org/tech/tra ... ervice.pdf
There is no lock out switch like one might see on later cars. You really need the kickdown switch to make it work the way it was intended. I suggest a 1952 Shop manual since the 1954 shop manual only references back to the 1952 one.
Is the transmission original?
Since you have gone to 12 volts it now gets more complicated. This site talks about some of the implications. http://www.fifthaveinternetgarage.com/t ... 2volt.html
(By the way, I was very impressed by the people at “Fifth Avenue Garage”).
I am currently replacing a 3 speed with a 3 speed/OD. I think I may have a 12 volt relay somewhere if you need that; but I have no 12 volt solenoid. I am staying with 6 volt all round.