The Chevy Longhorn was produced from 68-72, a fleetside with the longer bed for use with campers, which were being built 8.5' long at the time.
The AD trucks, 48-54, were available with a 9' bed on the one ton chassis, but were not called longhorns by Chevy. There aren't many of them around, and are not a desired truck by most collectors, due to the rough ride and low gearing.