Contrary to popular belief, when the frames are extended on big trucks (semi tractor, dump trucks, fire trucks) there is no welding done at the splices, and has not been since the mid 1990s. When the frames are cut (at a straight 90 degree angle), there is a piece of channel iron formed the fits tightly inside the original frame rail for each frame rail on the truck. The new rear section is butted up tight against the cut on the front section, and the formed channel is clamped into place and (4) 3/4" diameter holes are drilled in each the front section and the rear section (8 holes total, on each frame rail) and the inside channel is bolted to the inside of the original frame with (8) grade 8 bolts on each frame rail. Before the bolts are torqued, the square, level and straightness of the frame is checked, and then it is checked again after both sides are done. I believe on large trucks this needs to be certified.
I don't know where you got your info but I have to respectfully disagree that this is a normal procedure for cutting or splicing a big truck frame.
The procedure that was done on the last one I had stretched was to cut the frame at a 45 on one side and at a 45 in the opposite direction on the other side and weld it. Suspension and crossmembers were moved back the 16" I increased the wheelbase by and no fish plates. The job came with a pile of documentation for how and where it was cut, the welding procedure and a brake application timing analysis to make sure the extra 16" of airlines didn't take the brake application time out of spec.
I agree completely with the rest of your analysis. If you aren't intimately familiar with precision measuring, a square, a level and a welder, it's best to leave it to a professional.
Kevin
