Yes the long stepside could be had in 66 on a k 10. As could the optional spare tire fender. The tail lights should be lower. It could have came from the factory with out a bed. Cab and chassi

1947 1Ton stepside
1962 K20 Fleetside
1962 C40 12' flatbed
1963 C10 Stepside
1963 C20 Fleetside
1965 C10 Stepside (Daily Driver)
1973 C20 Stepside fire truck
1973 C50 Utility truck Houston Power & Light