Pull the "bullet" (the thing the speedo cable screws onto the trans)from the side of the Muncie (one small bolt and a flat bracket holding it in place, then just pull on the case). When the bullet comes out there should be a plastic pinion gear that comes out with it. When the trans spins, that plastc gear spins and turns the speedo cable. The gear may be missing or broken/worn out, so its not turning when the trans turns.
I bought a rebuilt muncie and hooked up my speedo etc only to find out the shop never put a speedo gear in the trans. Its kind of a pian to get to the area once the trans is in the car, but its not that bad.