As I posted above, the repro speedo cable for my '66 4-speed car is too short to use a retainer at the trans crossmember and is barely long enough to accept the retainer shown in Jeff's picture. I am not 100% sure that a '66 car got this crossmember retainer, though the evidence says that '65 cars did. There is a hole in the crossmember for the retainer so I wanted to add it to my car.
The Scott Drake repro cable I have is about 65"-66" long, which agrees with the length in the 1975 MPC (not sure if an earlier parts book would give a different length). I purchased a repro cable from Daniel Carpenter that is intended for 60's era trucks, p/n C5TZ-17260-Y. It is 72.5" long and will allow attachment at both the floor pan point and at the trans crossmember. Other than the length, it is identical to my other repro including the soft sponge type seal material at the firewall entry point into the interior.