No one has mentioned this, but on my '66 I had to remove the screws attaching the instrument cluster, pull the cluster forward and partially out of the dash in order to gain good access to the rear of the speedometer and unscrew the cable (reach in through the opening where the dash cluster was - not from under the dash). I know your dash is different than mine, but keep this in mind if you don't have good access when approaching it from under the dash.