I'm not sure what you are working on, but vacuum became obsolete in the mid 50's and no one has supported it in many years. If you have an Eaton 2 speed, an electric shift will probably work and they are vendor item. A Chev, Ford or anything will be the same. 6 volt motors are getting rare, 12 volt are pretty plentiful. There is a new and old style. You would want the old style which has a 3 bolt motor with holes evenly spaced. The later ones have 3 bolts also, but they are staggered. The new style has electronics. Any truck with a 2 speed from the mid 50's through the 80's should work. You will need a switch and a speedo switching gear if you care about the speedo.
