There were cab differences through the years. Wiper switch mounting changed slightly in '75 I think, then another slight change in '78. In '79, the VIN moved to the dash. In '81, the cowl changed to accept hood hinges and wiper posts used 3 mounting holes instead of two. The dash also gained speaker holes on each side of the dash, under the pad. In '84, the wiper switch moved to the column, so the headlight switch moved north an inch or so on the dash.
The different trans humps in the floor is a whole nother conversation.