6066 (1960-1966) GMC Truck Club

From the "web"..."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."
And this:
The first "all GM" factory 4x4's were introduced in 1960 when both Chevy and GMC went to a totally new chassis. NAPCO and its Powr-Pak conversion were left out of the equation due to the introduction of GM's completely redesigned truck line featuring independent front suspension on the two wheel drive trucks and a 4-wheel drive specific chassis on the 4-wheel drive trucks. This was the beginning of the end for the 4x4 conversion element of NAPCO. Though they did produce conversion kits for a few more years, their main business shifted to the heavier trucks, 1-1/2 ton and larger. A few later model specially built custom vehicles featuring the NAPCO components are still out there, such as Mr. Packy Pickrell's 1970 Chevy 1 ton NAPCO.

After the huge loss of the contracts with GMC and Chevrolet to supply conversion packages, NAPCO sold the rights to the Powr-Pak package to the DANA Corporation. I haven't found the date yet when that actually occurred, but we can safely assume it was after 1960. All documentation, archives, information and parts were transferred to DANA at that time. During this evolution, the association of the NAPCO name with 4x4's ceased to exist. The NAPCO 4x4 had vanished, just as it's associated history, archives and NOS parts supplies were absorbed into DANA."

Looks like you are correct about the larger trucks. Thanks for the clarification and info. I would say post 1959 1 Ton and up NAPCO's are really rare and somewhat valuable, but they don't fit in my garage.