I agree with the above - if someone replaced the cable somewhere along the line, could be it is too short at the head, and not spinning the speedometer. There should be a collar on the top so that the cable goes far enough into the head. Some of the universal replacements may not have that.
Otherwise, the heads are not too bad to work with - below is a good link - I used this to take mine apart and lubricate it. I also found out how to adjust the "clock spring" as I accidentally moved it - 1000 rpm is about 60 mph. Spin it with a drill and adjust accordingly. I fine tuned in the truck with GPS. Good luck with it.
http://www.chevytalk.org/fusionbb/fbbuploads/1320200988-Speedometer_Repair.pdf
