Holy Cow!! are those guys still there? I knew them way back in my air-cooled VW days in the 1980's. They started off as an offshoot from Randy's Stuff, another, even older, VW shop in Denver. Good to see there's still enough air-cooled VW business out there to keep them going. I still kinda dig those old cars--simple, reliable, elegant engineering.
My personal advise is to aim at a popup roof that can give you a minimal living space in case of bad weather .
Just my $.02, but I gotta agree with this. It doesn't seem like you're going to use it for any expeditions or anything like that, so you don't need much. Remember the KISS principle. With a simple pop-top, you'd have somewhere to go in bad weather. In good weather, you'll be outside most of the time. Put an awning on the exterior of the rig for hanging out under, but unless the weather's for sh*t, you won't be going inside except at bedtime. It's not a Troopy, there's never going to be enough room in there to live in it.
I'm thinking Step 1: try and separate the "high" portion of the spare top from the "original" part, trim it as necessary to fit over your top, then put a rubber seal (Westfalia, maybe?) around the perimeter.
You're not going to like Step 2, but then cut your roof (or source another one to cut) to allow stand-up space inside the back of the truck. That's assuming you can't save both the "high" and "original" portions of your extra top. If you can, all the better.
Step 3: Scissor hinges for straight up poptop, or Westfalia style hinges for wedge top. Gas struts (like a later Westy), or locking mechanism (like an early Westy) to hold it up. Latches to hold it down.
Step 4: Add wood or synthetic attaching "frame" for a tent to both halves. You'd do well to look at many vintages of VW Westfalia campers. Even the older ones will have good ideas for you. Especially 1960's, 1970's and up to the 1980's, the technology is pretty straightforward. Easy to duplicate and reliable. Go figure. German engineers.
Step 5: Consider separate left and right cots or "shelves" for sleeping instead of one solid platform. Not as cozy for you and the missus, and not as much room to spread out while you're sleeping, but plenty of space to get a good night's sleep, and allows for a gap between the cots for easy access. The cots could be made to fold out of the way easily. If it's just going to be for weekends, this solution should be fine.
Step 6: Remove your back seat. Use the seat and/or seat belt mounting points to mount a removable cabinet (or cabinets) for your camping stuff. I'm envisioning cabinets mounted along the sides of the back of the truck's interior. Could go all the way to the roof on the driver's side--not going to make a blind spot behind your head.
Obviously pretty shy on details, but, again, just my $.02.
Not sure if it's of any use to you, but I looked around in my files and found this: