speedo cable adapter

I'm pretty sure that the adapter that you are looking at is the same that's already in your tranny. My guess is that the tranny end of your cable has a small nut on the end of it and the 77 trans takes the big nut. You could use a complete cable out of a 66 or 67. The 66 -67 cable is somewhat of an odd ball because the tranny end has a big nut and the speedo end has a small nut. In 68 the speedo end changed to plastic and it snaps into the speedo. If you go to Brewers website and look at the very first cable for a 66,67 b-body you will see that they note that you can use this cable on 62 & 65 b-bodies with 66 and up trannys.

I hope this helps