Hey Zenoit,
There is a member on here named Toes- he and his father run Marcum Trucking out of Thurman, OH. These guys specialize in the work you are talking about, and about everything else that could possibly pertain to a truck.
Believe it or not, I added a nice big Alumi Bunk to my F650 day cab. I picked up the sleeper for practically nothing, thanks to a helpful tip from another member on this site, and from there, I had to find a different box.
You can not go over 40 feet long on two axle truck, so that is why I got the different box. That is where the fellas at Marcum Trucking came in.
Darrell contacted me after I emailed him about some questions I had about doing this, and he spent a good couple hours walking me through the old box removal and sleeper placement process.
After I got the sleeper on and bolted down, I took the truck down to Southern Ohio, where I paid Darrell and his father Bill, to put on one of their boxes, which was the right size I needed.
These guys did a phenomenal job, and I even lucked out and my new, used box had a functional lift gate. Most importantly, you could never in a million years, find anybody with more reasonalbe rates.
WG