It's nice you're in Japan. You can get all those parts somewhat locally. Here in Canada land everything is somewhat molested and a challenge to obtain.
My friend cut it off a 79 cab he had on hand. It was messy, off an ex mine truck, had holes drilled thru it i had to patch. I Just drilled a bunch of holes around the outside edge, did some grinding, clamped her up and plug welded. Paid $300cad for the panel. I would have liked a brand new one...
I did a thing like you too.
Got my springs re-arched and moved a leaf up front too. My front suspension was also bagged upon arrival. Fit 315 75 R16's good after that.
79 Series rear window panel
79 Series box
Rear seat delete & floor rebuild
140L fuel tank
Raptor liner paint job