If you can't find one to work this place can make ANY speedo cable you need. It's a cool old business too! You have to call them - no interweb nonsense for them.
Wyoming Automotive Co
205 East University Avenue, Laramie, WY 82072-3042
(307) 742-2196
As luck would have it, I was searching for for help on my radio and found info that a 64-67 tempest cable is supposed to work. And of course, what you read on the net is always fact, so I'm going to research it further...