My seats have are made from four 3" wide wood, just under 1" thick.
The backrest consists of two boards of the same size.
If you have an M35, you need five channels that fit into the channels on the bed, seven if you have an M36.
Each board is secured with nuts and bolts.
The fold-down "mechanism" is a set of hinges.
Each bench rests on five (seven on M36) fold-down legs made from U-shaped steel. The bottom is rounded so that the legs can slide into position when you put the benches down.
Two bolted on pieces of metal hold the seats "up" when they're not folded down.
The head (the piece by the cab) is made from the same wood. I guess all trucks originally had a cut-out in the second board from the top so that the driver could look out the rear window but I have also seen trucks without that cut-out.
The head and the side panels hook together. I don't have pictures of the hooks and eyes but if you need them, I'll shoot them over to you later.
Considering how much time it takes to build a set, you may want to consider buying a set and refurbishing it to get that "new" look. It may be cheaper, too.
If you want center seats also, you need ankers in the bed. Someone else is going to have to help you with that. I have never had a truck with center seats.