I was scheming on a similar project. Then I decided it would be easier to find a "furniture mover" type box that already had the attic built in. Then decided to downsize and build our own smaller box from scratch anyway. But, I did a lot of thinking on just the project you are working on.
My plan was to build an internal framework of 1.5x1.5 steel tubing in the front portion of the box to support the cab overhang and new front wall, attach the walls/roof to the new framework, then cut out the bottom part of the box. Then build the new front wall and floor of the cabover, and just slide the modified box forward and bolt it back down. My thought was that if I was careful in the removal process, most of the existing trim could be reused in a new location. For example the big aluminum extrusion on the bottom that you cut off would be the right size to just move up to the bottom of the cabover. Same for the curved pieces on the front of the box. However that stuff can be purchased new, any major city has a dealer for pretty much every brand of truck box manufacturer and they can order you all the replacement parts, which they have available for crash repairs and what not. I have a friend that works at a local dealer that informed me of all that when I was thinking this way.
I have another friend who build a toter similar to what you are doing, and he was also able to reuse the parts of the sidewall (cut off when you shorten the back) to build the new solid back wall of the toter. Worked good, everything matched up nice that way trim wise, including the "patina" as in every white truck is faded to some extent, and if you mix in new materials it will never match up right unless you paint the whole thing when you are done. He just shortened the box to the length where he could pull his fifth wheel trailer, but then decided to add the cabover later. That was a mistake, more work than it was worth. Should have done it your way and been less work and look better.
Using the 1.5" steel tubing for your internal framework would have the benefit of matching up to inexpensive 2x2 or 2x3 (laid flat) framework you use throughout the rest of the box for your insulation and wiring.
I ended up using surplus diamond plate curved corners and cast aluminum endcaps when I built my box. My stuff was surplus trailer hardware from Johnson's Surplus and Bontragers in Elkhart. A little tricky to figure out how to build the structure properly to work with the curved corners the first time, but worth it for a professional look.
You may want to also have your plan figured out for lower toolboxes and/or side skirtying at the same time if you want it to have that true toterhome look. For example that way you can run the same solid corner piece all the way to the bottom of the side skirt for a nice continuous look.