When Dodge first came out with the the Cummins Turbo diesel, a year or two before the big body style change, a Hot Shot driver wanted to put a 5 foot long sleeper behind his cab. Torch cut the frame, beveled frame, added fish plates, checked alignment about ten times, and welded it up with 5p stringer and 7018 fill. I had to add 5 ft. of drive shaft with idler, brake lines, electrical lines. Bigger job than I expected (as usual). Finding channel to match frame was not easy either. Well, that was one long One Ton Dodge. He drove it a year or two and asked me to take the five feet out. I did but only the welding part. He reconnected everything.
Like most, now I think I would pass due to liability, not to mention the pita of chasing all the parts and pieces to complete the job.