Idea..
Find a Peterbilt sleeper cab off a commercial rig, as they are fairly common.. Cut it in half vertically, remove the bed, run stringers from the front half to the back half, and enclose the gap in the middle with aluminum sheetmetal, at a minimum .032" thickness. Every 2 feet fab vertical "ribs", and rivet it all together, the less you use cherry max rivets the better. Cherry max work themselves out eventually. Solid "AD" rivets work better. Wet install with RTV from the local auto parts store, and tie in your stringers (horizontally) to your ribs (vertically). Reference Plan B supply for their aluminum framing. Their stuff is alum welded, versus riveted.
For floor, suggest .040" thickness floor pan, and a wood subfloor, tie into bed mounts originally on truck.