Keep all the parts. You have a complex problem and it's best to do what you are doing, asking questions.
Most chops cut out a section of the door and cab posts and weld back together. Going back the other way is X2 difficult. At the factory, the cab roof and cowl are joined (welded) together by a section (or just top or bottom) of windshield post and then leaded. Structurally integrity of the cab post is more important than the door posts. A body shop traditionally reconstructs or rebuilds at the same factory seam or joint. If obtaining an unmodified COE cab is not possible, I would find a 1/2 ton or big truck solo cab with floor, cab corners, bottom cowls that has rust through issues and use this as a donor. Even at that, these cabs can be hard to find in any condition. As an idea, not a recommendation.
https://www.ebay.com/itm/384853677647. However, this is work for a professional bodyman (autobody person) not a unexperienced DIYer. All of this is assuming the COE has the same metal thickness and contour. Most of the autobody repairers today may not have the experience and knowledge to fix an old truck so this needs to factor-in to your investigations.
Where I live, the door bottoms of old unused trucks, 1/2 ton and bigger, develop rust rot. The bottom weep holes clog up (leaves, pine needles etc.) and the rain water pools. It is common for them to need a bottom section replacement so a conversion to a COE door may work if the surrounding metal is still good. You still have to hand-fashion the needed COE pieces. Again a professional is needed.
From 1942 to 1945, car and truck production went entirely to the war (WW2) effort. You may want to ask questions at the military restoration websites. Expect to acquire burning eardrums and let the negative comments happen. Once you start, don't give up like some do.
