[dsraven]

did you get your parts cab home? how does it look?
if you are gonna do a cut and paste firewall and/or front section of cab, etc, you may wanna get the door hinge pillars good enough so you can hang the doors and get them fitting well. maybe some new or rebushed hinges as well, so everything is nice and tight. then cross brace the door openings and the cab from side to side, behind the doors, so things stay put. then you may wanna massage the opening so you can slip a windshield into the opening, with some short pieces of the old rubber placed here and there as spacers, just to ensure it is shaped like it should be before cross bracing that opening as well. do a few corner to corner dimension checks to ensure things are square. since the firewall is bent along with the cowl and windshield opening this could be a challenge to get the doors on properly but the last thing you wanna do is a bunch of work to get a new front section of the cab installed and then find out the windshield and doors don't fit because the cab wasn't symmetrical when you welded the new stuff in place. the other thing you could ponder is possibly getting some sheet metal bent up and replace the whole firewall with a new part, with no holes, and that allows you to build in as much of a offset into the firewall as you need to fit your engine choice. some guys even use a steel wheelbarrow shell for the offset because it is shaped sort of like you need and can be made to look like it was supposed to be there if you round the transition from firewall to shell.
anyway, just a few thoughts. my 57 cab was rusted almost as bad as yours. I didn't have the firewall or cowl damage though. those parts are also available in the aftermarket. I suggest you take your time if cutting and grafting another cab face onto your cab because these old trucks were not made to all the same exact specs like a modern vehicle can be. that means a door that fit perfectly on cab A may not even fit the opening on cab B. also, there can be distortion from heat etc when removing single parts from donors. take your time, drill out spot welds if you can rather than use the grinder or torch. watch out for the lead filler especially found, in greater depths, around the top of the A pillar to roof. also, some of the joints may be brazed which makes them harder to separate without heat and heat warps panels.

post pics, it keeps us all interested in your build.