In my opinion, forget the platform. Take out your back seat bases, fold the seat backs down, air mattress on top of the flat load floor. I am 6' and I have enough room to sit up with out my head hitting the headliner. An air mattress just fits like this and I have just enough room to spread out while laying down.

If this system works out for you, take out the rear seat backs and build a mini platform there for storage underneath. Then plan out storage on the sides over the wheel humps.

At the bare minimum, figure out a way to mock up your imagined sleep setup and see if you like being that close to the headliner before you invest more time into the project. For me, being able to sit up in bed is not a luxury I am willing to give up for storage space.