Truck Frame Stretching

I have a 2008 Prostar that we put 5'6" in the frame in 2014 it is 296" center of drives. We did it at my friend's shop and lined up the frame with string. It came out perfect and was only out a 1/6 of a inch on the rear axles. We cut it in half and added 5' 6" of 9400 frame. So there are four joints with a two foot plate over each joint. You cut the corners off the plates and weld the top, bottom and ends and not the cut corners. It allows it to flex which is how it should be done. I pull a 53 foot flat and can put 8'6" off the front with a 36" king pin. I have had 80 foot pipe on her, 47,000lbs of 75 foot beams and had 52,500 on the floor a few times. I have put over 600,000 miles on her and she is still holding strong. I gave my friend a grand and I had 1800.00 in parts.