Help! Need Advice for mating 64 speedo cable to 71 Trans !

You should be able to find that cable. I had one Barracuda that took a clip on speedo connection and another that had a screw on. 67 & 68's. Just get an accurate picture or take it out and look at it before buying to make sure it has the right connections.

Try having them look up a 67 and 68 barracuda to see if one has the screw on end that you are looking for. the trans end should work on your trans.