1969 C6 speedometer cable question

[Roger_1488]

So I am trying to find what thread size the transmission side of the cable is? I know the GM stuff is 7/8-14 on their big end, but what is this odd year Ford's? What I'm trying to do is adapt a Rostra cruise control speedo signal generator p/n 250-4160 https://www.rostra.com/manuals/250-4160_59A-02361G.pdf, and it almost looks spot on for my bump's big end speedo cable. This is just a cleaner installation than the hall effect sensor that just hangs out in the trans tunnel and waits for the four zip tied magnets to whir on by on the driveshaft. Any input?