The crossmembers are heavier due to heavy trans. They are also designed to unbolt so trans can be removed from bottom of truck. Brake line routing is different for same reason. Cab and trans cover different for bellhousing bolt access and trans dipstick access. I thought the e-brakes were the same. GMC and Chev linkages are different as well as steering columns and shifters. Carbs different as Hydra has auto choke and starters different for solenoid start instead of foot stompers. I am betting I have missed something but that is what comes to mind.