 Besides the four-wheel drive units NAPCO also provided winches, auxiliary transmissions, tandem drive axles, hydrovac systems, and dump truck bodies.From 1956 to 1959 the NAPCO Power-Pak option could be ordered directly from GM (an official RPO 690 was assigned in 1957) and factory installed on trucks with very few modifications to the original chassis.The 1955 4×4 NAPCO GMC or Chevrolet was a $1250.00 to $1550.00 optional add on.