hutch42 makes a good point.
Both the Sprinter and Transit cut-away chassis have a box frame made of very thin but very strong steel when shaped into a box. It's strength is in it's rigid shape which is compromised if cut and extended. Also, outfitters are not permitted to weld anything to the frame. All modifications like a rear overhang is bolted-on. So outfitters must work with the wheel base as-is from the chassis manufacture.
The E350/E450 has a "C" channel frame made of a softer steel that is much thicker and physically large. Welding to that type of steel has no consequences. Ford has very specific instruction on how outfitters are to cut the frame and extend it. Such Ford-approved frame outfitters do not void the warranty. Whether increasing or reducing the wheel base, extensions or reducers or replacement parts are required. Such parts include the exhaust, fuel lines, wire harness, parking brake cable, brake lines, and drive shaft. Given our PC-2350 uses the standard 158" wheelbase, it avoided all that mechanical deviation which was something I appreciated.
I was at a Chicago auto show a few years ago and took pictures of a virgin Sprinter cut-away and a Transit cut-away work truck. I made note how thin the Sprinter framing material is compared to my E350. I can understand why welding is not allowed on the Sprinter. The weld would burn right through the very thin steel.
I understand that if a Sprinter (and maybe a Transit) gets into an accident with frame damage, it is automatically "totaled", so if you are considering a used Sprinter or Transit-based motor home, be sure to run a Carfax and AutoCheck report. Any accident history requires extra precaution because if twisted or otherwise deformed and not diagnosed properly, you will have bought a problem.
CLICK HERE to see 16 pictures I took at that show.
