I was just getting ready to mount my cab to the frame (scheduled to happen tomorrow) and thought about checking the speedo cable. Stuck the end into the tranny and spun the cable. It should do that, so I dug out a new cable that had been hanging in my dad's garage since about '96 (that was to fix a speedo issue on the car I bought for my daughter for college - which turned out to have an electronic sender and speedo.

) So it got put to use today on the '51. Stuck the end in the tranny and no spin like it should have been.
A new cable like Wally suggested might be a good idea.