You're not crazy; you're creative!
I don't kow much about stacking the cars, but I can tell you a bit about an "extended sleeper."
Depending upon the HDT you buy, get one with a factory installed sleeper (bunk). From a local scrap yard, or from E-Bay if you have to, purchase another identical sleeper of the same size and brand which has been stripped off a tractor. You won't have any trouble finding one, becase "day cab" tractors are worth more money on the used market than sleepers tractors, so the bunks are just stripped off and usually sold for junk.
It is certainly more complex than we have space for here, but basically, cut the back off the sleeper which is mounted to the tractor; cut the front off the junk sleeper, and huck or rivet them back together. Everything will align, although some fiberglass work on joining the roof halves will also be necessary. TIG weld the side support beams, and join the floor. We even added windows.
I used to work in a heavy truck fabrication shop and we did this several times for customers who wanted a "super sleeper." They we all done to Peterbilt 379's with the 63" bunk, and they look fantastic when done properly. Do all of this before mounting the 16' box on the rails, as you'll need the access space to work. You could use the mounting ring on the rear of the back sleeper and align it with your box to make a complete walk through.
Hope this helps.
Highway OPie (Speed Gray) Grand Rapids, MI: 2003 Peterbilt 379 Motorhome; 550 hp 6NZ Caterpillar C15, Eaton-Fuller 18 speed transmission, 3.36 rear; 63" sleeper and 16' Morgan box.(firstname.lastname@example.org). There are many Peterbilts, but this one is mine!