speedo cable

Another member posted that the cable he got from rockauto was longer. Correct length for A-body is 63 inches. Doesn't sound like a big deal but... There isn't a good way to route a much longer cable. The correct length cable is available from some vendor.
As for attaching a screw on type cable to a clip on style instrument or visa versa,
There is a plastic coupling for plastic tubing that will work. I saw this done once years ago. The end of the cable sheathe was modified. I'm thinking I would start with a cable that attached properly at the trans end. A cable for a 67 model A-body might work at both ends. Good luck