The top radiator support on the GMC goes over the top of the radiator. That forces the filler neck to be mounted to the back side of top tank and the petcock is also on the backside bottom. GMC uses a fixed radiator cap(no pressure relief). Original GMC radiators have a pressure relief valve on the backside of the toptank and the overflow line runs from the pressure relief valve neck across the top and down the side. The Chevy has a filler in the top of the top tank and the overflow runs off the filler. Chevy uses a pressure relief cap.
