John - that "3 window" GMC cab was a factory modification of the usual big window cab used only on the largest trucks, they're scarce but otherwise same as any other 5-9 cab, and other than the dash all 5-9 GMC and Chev cabs are interchangeable, big or small window .... changing the dash isn't a cakewalk tho, involves drilling out many pinch welds right on the windshield flange, good luck w/ that!
Bill
