GMPartsGiant.com offers the wholesale prices for genuine 2015 GMC Sierra 3500 HD 4WD parts. Parts like Engine To Transmission Mounting are shipped directly from authorized GM dealers and backed by the manufacturer's warranty. Parts fit for the following vehicle options. Engine: 8 Cyl 6.0L, 8 Cyl 6.0L Flex, 8 Cyl 6.6L Diesel. Model Code: 3500 CREW CAB LONG BOX, 3500 CREW CAB ONLY, 3500 CREW CAB STANDARD BOX, 3500 DOUBLE CAB LONG BOX, 3500 REGULAR CAB.