When I worked at that crane place, one of the older guys spent a few weeks stretching a truck. He turned a day-cab bobtail into a straight truck. After stripping it down, he basically just cut the frame in half making diagonal cuts (to maximize the surface area for the welds) and moved the back half about 15-20 feet back with the overhead crane, then a flatbed truck delivered the extension pieces for the frame. We welded the extensions in, used a couple of 4 foot long scraps to overlap the welds and bolt on for reinforcement, then spent another week or two re-doing the wiring and air lines and stuff. The custom drive shaft was made somewhere else and delivered to our shop. After it was stretched out, I put a flat bed body on it and another guy helped me mount a 40 ton telescopic crane to it and make a big H-shaped boom rest on the tail. I'm not sure how practical it would be to stretch out a brand new truck vs. ordering one already that long though. This was something the owner of the company got cheap at an auction and decided to do this to it, then sell it. So the labor involved was not at retail markup, it only cost him what he paid the employees. The labor costs is probably the only thing that made it practical.