I had a similar problem. The speedo needle would actually go down as speed increased. Replaced head, cable, same thing. The 3spd trans has a spacer to pinch the yoke to the speedo gear riding on the output shaft. In my trans, this was loose, so the speedo gear was not always spinning at the speed of the shaft. Took apart and replaced spacer on shaft. Worked ok. This was also on a 54 3100. I just got it done, so it is still fresh in my memory.
