I am rebuilding a 1976 GMC K2500 that needed a better cab. I found a 1986 cab and i'm in the middle of converting it to a 1976. There are several differences in the two cabs. Here are some pictures that may help you with your project.
https://www.kijiji.ca/v-cars-trucks/owen-sound/1983-gmc-jimmy-frame-and-bare-tub/1414956766?enableSearchNavigationFlag=true
1983 Jimmy frame and bare tub...
Carefull, ease into it when you pull out pass on wet/icy roads.
I put my foot into it with the dually, ended up sideways around the corner. Scared the crap out of my wife.