Alright, after inspecting a pop up camper and doing some thinking and "planning", I've came up with the following.
jage - your right about the size of the popup, its too wide and the construction is way too heavy. I will make one out of aluminum that is lighter. As far as cutting the hole, I will def. start small and take a little bit out each time.
Thinking about wrapping the hole with a "U" shaped steel (.06" thick) and welding to entire perimeter of inside cutout to strengthen and give me a platform to lay the top portion of the beds on.
The canvas is way too tall (as noted by Dave), I'm 6' not 10'. Also, there was the door cutout to contend with, this isn't shown in my "rendered" drawing. I'm looking into doing the canvas myself, using some Sunbrella canvas and maybe some marine grade plastic for a few side windows and some screen mesh flaps along the "penthouse" fold outs on the 4' front and 5' out the rear, and 4' out the front. Back is for adults, front for little kids/babies.
Lifting of the unit will be done in all corners angled with exterior of van walls. Two lifts behind front captain chairs, and two in back corners. I'll find some surplus 12V linear actuators...
Once I get a chance to measure the van I will design up the lid and lifting mechanisms and start putting some dimensions on everything with the roof. Then see about configuring the floor layout. In the mean time, I'm searching CL for parts that I can use in the build.
If this sounds low budget, its because it is. I don't want to put thousands of dollars into a '76, also the wife isn't quite sold on taking this proposed contraption to a camp ground and showing it to my family.