Now, I just gotta say it again.. There is no reason to pound ANY holes thru the side walls to hold your interior walls in place. 3/4" Oak, bonded with a good adhesive and secured with a few stainless screws into those uprights, and you can build inside of those wooden uprights without any THERMAL TRANSFER to your desired thickness of insulation.
Nothing worse than frost growing inside an insulated box because of screw heads bringing cold into the heated moist interior air. You got enough other problem areas regards insulating with a step van. And use LUAN.. its lighter and flatter than miost plywoods, unless you get cabinet skin stuff.
If I were doing another truck, I would use a combination of traditional fiberglass and foam panels in the side walls. My first 14' truck I did all fiberglass. My second 14' was all Foam panel. The foam panel was a lot LOUDER inside... After all, if you knock on a panel, you hear it... if you knock on Fiberglass.. well, you get the idea. So, Glass for better noise cancelling, Foam for higher R values. Just keep 4-5" of only foam along the bottom edges near the floor. I actually left a gap at the bottom edge, covered with a removable baseboard that could provide inspection opportunities or the ability to pull a wire in later on. You'll probably notice when you get to doing your walls,,, that big GAP along the top edges which also makes an awesome wire/duct run for running wires and such. Just need a trim board that can be removed if necessary.
Don't forget a good plan for the floor insulation. My first truck had none down there, only 1/2" plywood and carpet. Second truck had 3/4" PolyISO under 1/2" plywood. That was night and day, but given another truck with 84" side walls, I would probably do 1-1/2" under 3/8" plywood. It's not a simple deal to insulate under the floor. It could be done, but the amount of unavoidable thermal transfer would be hard to make it worth it.
We had a 16' Grumman where I worked last with the 4BT... Truck is still there in the weeds with a bad pinion bearing. I could probably pick it up for $2000 or so, but I want a 12' Dually 6' side wall truck if I'm going to do another at all.
Still, your pics,.... recalling how dirty I could get crawling around cleaning up a frame with the needle scaler... Being able to pull a tranny from the top.... Putting a new intake and carb on during a snow storm with the furnace running...Ah... the good old days ! They were wonderfully made. I have a lot of literature I collected over the years. Grumman sales lit said it was a truck "made to run for 15 years"... shoot, many are 40 years old and still going.
I'm surprised you fixed your lower control arm... Same arm as the Chev 1 Ton Vans. Both mine had really iffy (rusty) spring pockets so I just always replaced them and the pivot shaft. Cheap parts ! Yay !
That 4BT NOISE would drive me nuts. If I had to own it, I'd be buying a whole lot of Mass Loaded Vinyl Barrier and figuring out how to knock it down. My Gas trucks were like Cadillacs in comparison.