I've used my long wheelbase GMC van for hauling cargo and for adventuring.
I have e-track in the floor, and would not hesitate to screw lengths into the wall bracing, though suggest Riv-nuts or through bolting where you can, to reduce/eliminate vibration loosening self-tapping screws.
If you're putting ply on the walls anyways, it depends on how thick as to whether you'd screw e-track to it, or go through it into the wall braces.
As mentioned above, several attachment points along the length is advised. If attaching on top of plywood, I'd have those points at the vertical braces go into metal with Riv-nuts, and the rest into the ply.
View attachment 653885
Another option is to build side cupboards like above at the height you want your bed platform to be. I have two simple folding bed platforms, each as wide as a sleeping bag, with 3" upholstered cushions. I usually just carry one, though bring the 2nd if someone else is coming along. Each bed platform can go along either side lengthwise, or across the width anywhere along the lengths of the side cupboards, supported in the middle by a crate the same height as the side cupboards or by two crates if setup along one of the sides.
Across the width like this:
View attachment 653888
.
My side cupboards hold a ton of gear, straps for the e-track, and tools, etc. They are flared out to the rear to full width of the doors and narrow slightly to 52" wide between wheel wells and beyond. That way I can funnel a pallet or sheet goods or whatever into the cargo area without banging up the ends of the side cupboards.
A lot of vanners want their bed platforms larger and going across the whole width more permanently and often a bit higher to provide a 'garage' of sorts underneath accessible from the rear doors.
I opted not to do that because I like keeping my bike in there, I like the more walk-through aspect possible in a van, and like that I can fold up the bed board and stow it and the cushions just the other side of that half-bulkhead, even when on the road. With this arrangement, it's a quick and easy switch from adventure van to empty for work or helping someone move, etc, and still retain considerable storage space in the side cupboards. They have simple lids with finger holes. Worked finest kind for years.
If using track on the walls to support a bed platform or to secure other gear, there is also
airline L track that is favored by some. If you already have e-track, you're all set.
.