Bill, Did the 73-76s use rod or cable? Can you do a little research on the ends and lengths of cables to see if one from the 80s or 90s can be subbed in?
The exact length of the cable is not critical because it is held in place by the bracket arm, and has to be adjusted for fit before clamping down.
Todd, Try looping the cable until you find a replacement. As already posted try salvage yards, pull a couple extra while you are there.
John