Your find is remarkably similar to mine in respect to miles, rebuilt engine, and box size. The only significant difference is mine has an auxilliary axle, which is of little use to me and will stay stowed forever because I doubt I ever load Moby sufficiently to justify lowering the axle. In fact, if there's a market for it, I would sell it.
Naturally, everybody has a different vison of what makes the perfect truck, but there are some similarities, e.g. cooking, eating, sleeping, working, and sanitary set up. For example, I like your ideas for the garage and sleeping arrangements but I wonder, what are you planning for SSS (shit, shower, and shave) facilities?
I presume the plan is to reuse components from the 5th wheel find, but where are you planning to install toilet, shower, and sink? I wonder because I didn't notice them in your drawing. Speaking of which, I had to zoom the browser to 400% to see much of anything, how about emailing me the .sldasm drawing so I can eyeball it more closely?
email: info at sanfordmachine dot com
Meanwhile, I've different plans for Moby, my conversion, because my garage space is for a modeling workshop instead of a car. That said, I would love to figure out a way to load a car because I am into them too:
http://www.hobbytrading.com/private//62gto/
Does your truck have a lift gate built onto the back of the truck? Mine does and I obtained a piece of 8" aluminum channel for loading my Harley (to have wheels on location), but this won't lift a car. I am thinking maybe ramps, but they would have to be very long for a street car. Regardless, this is low priority for me while ramps are very high priority for you. That, and your kind of vehicle has greatly reduced overhang fore and aft, which makes getting it in and out of your truck far easier.
Finally, I think I've sussed out the SSS facilities for Moby - thoughts in the Moby build thread but please keep your thread going, it looks like a very interesting project.