You have been told correct in that it uses a cable from the transmission to the speedo. That cable at the transmission end is much different than later cables and uses a hard-to-find gear. Do you have the correct speedometer gear? Tire size equivalent to the original 13 inch size?
Check out the Speedhut gauge line. Many of their speedometers include a tach and they are - as far as I know- electric. You will probable need to modify your cluster to mount it though
