I had a 1970 that was built by a company in Grants Pass, OR. It started as a standard length K20 and they shortened the front of the box and added the rear cab extension, so it remained standard length and had a short box. It was only a two-door with a split front seat, and the passenger end of the seat folded forward for access to the back seat.
I LOVED that truck, but sold it about 25+ years ago and I'm still kicking myself. I managed to locate it again a couple of years ago, and one of these days I'm gonna pay the guy a visit and see if I can buy it back. It would make a great expedition rig.
Just one of several vehicles I've sold that I regret selling. Another was a 1976 FJ-40.