I'm 6'3" and yes, I fit fine (quite comfortably, actually) for sleeping in the back; however with enough gear to support a longer (more than weekend) trip or more than two traveling light, I think you're going to find it less than workable.
For it to work for me, I have to have the seat bottoms (and headrests for the backs) out and the front seat all the way forwards ... then I put the 8gal Rubbermaid Action Packer that I use for recovery & emergency gear in front of the folded seat back. It winds up at the perfect height to extend the platform provided by the seat back and rear cargo area. If you've got the fold flat front seat like I do, it opens things up enough to not feel claustrophobic and gives a nice little shelf for my glasses, phone/alarm clock, headlamp, hat/gloves or anything else I might need quick and easy access to.
The problem, though (since I haven't made a dedicated platform to preserve any storage area), is that then you're sleeping in your main storage area and need a place to put anything that's there when you're not sleeping. This means it either winds up outside or all piled on one side of the truck if it's just me.
I suggest if you're serious about not using a ground tent or trailer, and want to set up the X for a longer trip, make like you're getting ready for it and see how much gear you've got ... heck load it all up, and try to see what the "getting ready for bed" routine will be. This will tell you whether it's really feasible for you, and if you still want to pursue it, what you need to do (as far as designing/implementing integrated storage under - or over, like an elevated rack - a sleeping space) if you're not willing to put stuff outside while you sleep.
