you say it is a c20,,,a c20 is 2wd..if it is a 4wd,it is a k20.....this is a 4wd,,right...and you want to know how to adjust the transfer case linkage,,correct?
this should help you
GM Full-Size Trucks 1970-1979
New Process Model 203
See Figure 1
FIG : Fig. 1: New Process models 203 transfer case shift linkage adjustment
The 203 full time 4-wheel drive transfer case is the only one on which linkage adjustment is possible.