Find this all to be interesting.
Such a short lengthening of the frame doesn't produce much extra box length/ capacity for the work involved to stretch a frame.
The frame area between the torque tube and the rear spring hanger, must be highly stressed on the pickup trucks.
Toyota in the 1966 and 1967 longbed versions plated that area with a c-channel configuration welded to the outer, top, and bottom part of the frame. The rear cab mount is hot-riveted on the outside frame there, while the bed mount is simply welded. In the later longbed trucks, the frames have a much smaller plate reinforcement welded inboard between the torque tube and the cab mount(riveted). Perhaps in their redesign of the frame, the engineers' research had found that to be the spot under the most stress.
Two thoughts on why your trucks have such a relatively short frame extension. First, was there at the time an existing trayback bed used for a different application, that needed the extra frame length to fit your Toyota?
Or--as I found in the Netherlands--is there a governmental taxing authority that taxes "long" wheelbased vehicles differently for importation, or even restricts their importation if not of a certain length. Ofttimes local politicians put on draconian trade regulations to appease a locally-produced domestic company, guaranteeing local jobs and moneys in their jurisdiction. Or they feel that a certain sized vehicle will be utilized for a different purpose and tax it differently.
Here in the States, Toyota was restricted years ago from shipping completed pickup trucks with their beds mounted at the factory. Back even then, the top US manufacturers were worried about the competition from the imports.
Could you post-up the length between your two white chalk marks to see just how much extra length all that welding actually produced?
