I just can't see GM re-tooling for a special length bed and frame, only to offer them in,apparently, very small numbers.
I dont know for sure but I believe the Longhorns used a bolt in extension in the front of the bed and the frame was used on other production vehicles as well.
(rambling)
My point is that, although relatively rare, Longhorns were produced in large numbers, yet they did not receive longhorn specific parts other than easily produced badging and the bed filler.
This guy would have you believe that GM tooled up for this truck, and only produced a hand full.
What I can see, is somebody not realizing how much and where to cut the frame to make a SWB, then havng to mod a long box to fit his f@#$ up.
Whatever