I've seen very few GMC's that still have them. Originally there was a heatshield mounted on the exhaust pipe. ( Not sure they were on all years. ) I made my two out of 1/8th inch aluminum. 28" long, and 6.5" tall. Made the in an el shape. Attached them to the bottom of the floor with 3 screws. They greatly reduce the heating of the fuel in the gas tanks. Bob Dunahugh 78 Royale Iowa