I have no evidence to back this up, but I believe there MUST be a different cable from 70-71, to 72-74.
My history... My 70 gauges looked like crap, and the tach and clock were dead meat, so I replaced them with a set of later year gauges until I could afford to have the originals redone. Suddenly, the speedo wouldn't work.
I took the cable out of the end that plugs into the speedo, and wrapped a couple of layers of masking tape on it, and it was fine after that. Go figure...
What I'm saying is, maybe that cable is just turning inside your speedo housing and not touching anything. Other than that, I agree with Chryco. Was your speedo ever working? It could be the gear inside the tranny causeing you problems, or maybe it isn't "clocked" to the size of gear it is??
Mike