Most of the parts you are looking for fit a 48 -53 Dodge truck (any size). That will improve your search, look for 48 truck parts as most listings will be found under the 48 search. A few years ago there were several places you could get some replacement sheet metal, but I don't know how many of those places survived Covid shutdowns. A search for 48 Dodge trucks should turn up a lot of options.

The Dodge truck cab changed quite a bit in 54, and then again at mid year 55 the cab was updated again. By 58, few parts other then mechanical parts will interchange.

The flathead 5 came in two verities, a short head version and a long head version I believe the short version is 23" and the long version is 25". The performance intakes have to be for the correct head length, measure your head's length. Both motors were put into trucks from the factory as an option and there has been a lot of years since your truck was new. Most of the exhaust split manifolds I believe are interchangeable (I could be wrong on the exhaust as well). Do a search for the Dodge flathead 6 performance parts, some stuff is still out there.

The old Dodge military trucks were based on the 39 - 47 cab, but a lot of non-sheet metal cab specific stuff crosses over. www.vintagepowerwagons.com has a lot of stuff available that will work on your truck, they repop a lot OEM parts.

You will find that one company (there were 3-4 companies repoping the 48-53 truck parts) will make a run of a specific part, and everyone sells them until they are gone, then when the demand gets high enough they make another run, and everyone sells them until they are gone. Part availability will depend on how your timing for looking for parts falls on when the last run was made. Rare parts without much demand do not get repoped at all so you are limited to what OEM parts still exist.

The 51/52 Dodge truck was about the highest market share that Dodge trucks ever enjoyed until 1994. There are still a lot of these old trucks around, but nothing is cheap for them.