If I were to do something like this I would never chop into the cab or chop up another cab from a donor truck. To me the idea of being able to unbolt the cargo bed, van body, etc from these trucks and slap whatever you want onto the chassis is just plain COOL. These trucks are like Ruger 10/22's.
With 12' of chassis and a 5 ton weight limit the possibilities are endless! If I ever decide to turn my M220 into a crew cab type comfort vehicle made for multiple passengers I would build a 12" long enclosed cab that could be bolted onto the chassis via the existing mounts. It would sit flush up against the existing cab so it would look like a 1 piece crew cab though. A window up front for driver communication or even a comm box would be awesome. Custom passenger seating for 12 people with all the creature comforts you can think of. All wiring would be in the form of a main harness that could be easily detatched from the main truck harness in the event you wish to remove the crew cab body and put your van body or cargo bed back on.
Dang....I need to get me a good forklift for more projects. On a more serious note, I plan to be removing the cargo bed off my 455 Olds powered M220 and putting the proper shop van body back on someday. I'd like to make a mobile gunsmithing work shop out of it. I have been doing my best to keep it as original as possible looking too. Still a gasser, still an auto tranny, still 100% GM, just has lots more power.