NAPA still has many parts available for that truck. My son has one with a grain box in good usable condition. The window glass can be made by any good glass shop if you have the old one for a pattern. But the doors and glass are the same on the travelalls and the pickups of that same vintage (1963 - 1968).
IH trucks (Navistar) does not have anything of that vintage unless you find a small town IH truck dealer who has been stocking this stuff for 50+ years.
The brakes may be your tough problem. The hydrovac units below the driver's seat under the cab were notorious for crapping out. But they are rebuildable. The brakes have 2 single piston hydraulic cylinders, one at the top of the wheel and one at the bottom. They are tied together with a well bent steel tube. That tube may be hard to find, and is not likely to be DIYed without kinking.
If it has the V8 engine 304 or 345, the engine timing is done from the #8 cylinder. The cylinder #s are cast into the intake manifold.
Good luck with your project!!
Paul in MN