As early as 1942 NAPCO began building four-wheel drive assemblies to be fitted to Ford, GMC, Chevrolet and Studebaker vehicles however NAPCO is primarily associated with GMC and Chevrolet pickups. From 1942 to 1956 4×4 GMC and Chevrolet trucks could be ordered by the government and civilians with a NAPCO Power-Pak kit and the kit could be installed later. The retail price of Napco Power-Pak was $995. This option raised the price on a new two-wheel drive truck from $1,548.96 to $2,796.96. The kit was shipped in a crate measuring 80"x30"x26" weighing 1,410 pounds. In a matter of 3 hours with as little as 4 holes drilled in existing chassis a truck would be converted into a "Mountain Goat", a "full sized truck that will climb steep inclines with ease". One feature was the "shift on the fly" rubber mounted transfer case with a dual-range option. There were companies that installed these upgrades for the previously mentioned manufacturers. 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. The 1957 Chevrolet and GMC 3100 4×4 price was a bargain at $2549.00 compared to the earlier add on kits. In 1960 NAPCO and GM parted ways when GM redesigned the front suspension on their 1960 pickup line so that it wasn't easily compatible with the existing Power-Pak kits.
Prior to 1955 NAPCO Power-Pak conversions were done on 3/4 and one ton GMC and Chevrolet chassis. The Pre-1955½ ton chassis used an incompatible "torque tube" drive. In the fall of 1954 GMC and Chevrolet changed the 1955½ ton pickup and Suburban models to the compatible Hotchkiss drive. Today these trucks are still considered to be very versatile and durable 60 years later and are considered to be collectible by NAPCO enthusiasts.