[SS Tim]

Nice start there Leon. Didn't see your earlier thread. My longtime interest in 4x4 converted C30s and converting Longhorns has dug up some interesting trucks over the years. My focus has been on the true bolt-on field installable kits from NAPCO and Marmon & Herrington.
This method of stretching a K10-20 frame is one way and while quick consideration needs to be given to its ultimate use.
A stock C20 frame is about 8% stiffer than a K10-20 frame. A Longhorn frame is about 45% stiffer that a K10-20 frame. I'm gravitating towards a C30 157" frame for my build be it with a 8 1/2(133" wb) or 9'(147"wb) box. It is about 100% stiffer than a K10-20 frame before I cut it down. So if you or anyone planning a heavy camper or similar high bending/torsion load might want to take this into consideration. Even a C30/K30 designation is questionable if you are retaining a HO52 rear axle in the process. Remember the CK20 GVW tops at 7500lb and a C30 pickup is 9000lb.
Finally the population of 4x4 Longhorns seems to be growing and in a good way (suspect Dieselwrencher's progressive modification build is going to become a Crew Cab before its ever a driver). Here is a conversion from several years ago that you may not be aware of. It is a 1969 C20 Longhorn and has a 4" lift suspension and a 2" body lift with 235 85R /16 " tires.