There are trades offs w/ everything...Boo. ha ha.
We built out a custom slide in camper shell last winter, our first new, "non-reno" build so in essence we had a clean sheet of paper to start with. It was delicious but also a bit daunting in a way as all of the design decisions we made up front, we would have to live with for a long time! Like you are planning, the camper was built to our dimensions and specific to our bone stock GMC 2500 HD longbox, regular cab 4WD. Like you also want to do, our design minimized the space between the cab roof and camper overhead.
Here are a couple of our numbers;
-with 265/70x17 10ply tires, the exterior height is 9' 9" ("sliders" to be added in the spring will bring it up to just about 10' exactly)
-85 inches wide (WO jack brackets),
-32 1/2 inches floor to ceiling in the bed area and
-54 inches of interior bed (East/west) width.
We used an "off the shelf" 4 inch hard foam mattress on top of a venting layer of 3/4 inches. At our "average" body heights (5'10, etc.) the interior bed area height with mattress/spacer is just perfect for us. With a north/south sleeping arrangement, these clearances might be a bit claustrophobic. The East/west setup for us is a first and we are actually really liking it. We have a very "open" interior design so it works well for us. Plus at being "north of 70" in age, most of the horizontal activities now involve reading and sleeping.....
For context, here are a couple of pics from a few nights ago.