pretty silly question, but did you pull the old cable out of the sheath to make sure its intact? The transmission speedo gear seldom breaks but is easy to replace if it does, on the other hand one of the first things I replaced in my '57 with a sm420 was the cable. clean it up and lube before reinstalling and make sure it spins freely in the sheath as they get abused, crushed or kinked with PO's changing out motors and trannys.
