Although the following photo shows a hydraulic slave cylinder, most mechanical clutch linkage shares a similar retaining spring between the fork and the linkage, specifically to prevent the pushrod from falling free from the fork socket.
The following diagram shows the retaining spring (item #14 just above the pushrod) on a 1967 Chevelle setup. I would suspect your truck should also have one:
http://www.classicchevy5speed.com/fi...d_push_rod.pdf