I have separated the two different kinds of cabs at the belt line for the purpose of taking the rear panel from a three window to fix a five window. From the looks of things it could be done as the cab is made from two main assemblies. The roof assembly portion includes the A pillars, the roof and is joined at the belt line to the lower half assembly. There are three or four bolts below the rear window that can be accessed from the inside of the cab. Sometimes these come out easy and sometimes they just spin. If they spin - your screwed and will have to burn them off. Your primary challenge will be to remove the gajillion spot welds that are also joining the two halves. I may have underestimated the number of spot welds though as some cabs have more. The fun will be that you get to do this TWICE, once to each cab. You will then need to remove all the lead that is filling the seams on the A pillars and drill out those spot welds also.
When you start putting it back together you will have MAJOR fun getting the door openings aligned properly.
You might be able to make some money off this though as I for one would buy a ticket to watch someone try this operation. I would provide tons of moral support but you won't catch me drilling the welds for you. Been there.
Once you finish this your level of experience will meet or exceed most other stovebolters.
On a serious note - Fix the rust on the 5 window can and sell the three window if you just have to have the corner windows. I would keep the three window cab myself.
Good luck and let me know where I can buy tickets.
