Stoked for you!
The tires I’m using came with the truck; Cooper Discover S/T. I haven’t noticed any problems at all, but haven’t really paid attention to them.
Regarding building the back platform, the first thing that **** did was to ask me what I want to use if for, so, what do you want to use it for?
My guess is you’ll be doing the same things you’re doing with your current truck, (hunting, fishing, skiing, Norris hot springs) with a little less emphasis on long term comfort and a little more emphasis on gas mileage and nimbleness. The following recommendations are based on that assumption.
If I were single, or planned on sleeping singly most of the time and wanted constant “work/head” space, I’d divide the bed on the long axis, not the short. Not sure if I’d divide it exactly in half; check how much space to sleep you want/need, but it’d be nice to always be able to sleep and to throw in a deer carcass or ski poles etc without messing up the bed.
I like the aluminum legs, even if they are a bit more expensive. I’d try and find something a little thinner (and lighter) than the 3/4″ I used. If you use angled metal you can probably put a couple slats across it and get away with 3/8″ wood. Not super sure about that, I just always want to build with the minimum amount of material possible both for cost and aesthetics.
For taking it in and out and fastening the wood to the metal I like the wing nuts, but if you’re not going to pull it out once you put it in, go with a locking nut and loc-tite combo or something comparable. ****‘s idea of making sure if you roll nothing knocks about is a good one; everything should be securely fastened.
I love the pull-out boxes underneath, they make it very easy to stay organized. Maybe get three smaller instead of two larger and have a dry clothes box, a kitchen box, and a wet box. Make sure they’re secured underneath the platform.
5″ is too thick for a mattress, I’d probably cut it down to 3″, check with your local foam dealer. If you come through SD on your way back there’s one right down the street from our place and they did ours in about 3 days. Mine is super comfortable but a little thicker (and more expensive) than necessary. For hold downs I’d see about extending the metal frame to form a lip on the center side of the platform to hold the mattress in.
As far as a cap, if you can, get the one with the fold out screen windows and seal it up against leaks…mission critical in your neck of the woods. I’d also look into one with a slightly raised back end for the extra head space, although that’ll affect gas mileage. Check for lights in there.