Hi Tom,
I'm guessing you are from the northeast , maybe the New England states ? Some folk in that area of the country refer to a PU truck shell as a "CAP" ..
Different areas of America have different names for PU Truck covers.. CAP, COVER, CAMPER, SHELL, TOP, etc.
Where I came from (eastern NC/SC) we called them "Camper Shells"
But the short answer is YES.. you can "camp" in a 6 1/2 ft bed. if your CAP is cab height it simply means that you must remember not to stand up when the 2:am pee call comes around.
I've had a couple of short bed trucks and one had a cab height camper shell ... 19" + 23" side rail = about 4 ft inside height
The other truck had a 36" camper shell ... 36" + 23"rail height = about 5 ft inside height.
The 36" was nice because I could use the porta-potty without bumping my head on the roof.
I also had a couple of PU trucks with the standard 8 foot bed that had a rail mounted 60" camper shell … 60" + 23" rail = 83" inside height which allowed me to stand up.
I've only had one factory made "Slide-in" camper and it was nice but I didn't like it because I could not take it on and off as easy as the rail mounted camper shells.
As for installing a roof vent.. is the roof surface completely flat or does it have raised ribs or other uneven roof surface. You can install a vent on either one, but if the roof is not completely flat you will need extra caulking to fill in the gaps around the edge of the vent flange.
Don't forget to run a wire from the truck's battery or other battery you install.. connecting to the tail light circuit or the cargo light circuit won't work because you will need to keep the head lights or cargo light on all the time to supply juice to the fan.
Hope this helps Tom and good luck with your project and proposed trip to the southwest.
John