Try posting on the site below...I am not sure on the older C-Series. Is there two 3/4" holes on each side of the tool box towards the upper rear? If so, that is where the rockshaft goes to transfer motion from the cable to the rear clevis hitch. Some just run it from the tube to the clevis hitch, that is how it was on older tractors than yours. The cable hooks to the lower arm (lower hole) on the tractors lift mechanism. The tube for the Auto's is bent and the 8-speeds are almost straight. There are 2 clevis hitches on EBay that show all the parts.
I know how it works along with many other collectors. I have several 1978-1984 C-Series with this setup. Many new folks in the last couple months have gone thru the same thing as you...
Red Square:
http://whtractor.15.forumer.com/