[dsraven]

like said, install the doors and get them to fit. check the hinges to make sure they are not worn out. you can rebuild them yourself if not bent or rusted too badly. anyway, install the doors and then get them to fit properly. themn brace the cab so it can't flex. when I did mine I installed a horizontal brace across the door openings inside the cab above the floor so a 2x4 would fit there and still be above the floor. that way you can lift or support the cab using 2x4's run through under the brace. this would be handy for you if you need to do floor work. the 2x4's can be long enough to extend far out and be set down on saw horses. that gives lots of room to work on the floor. I also ran a horizontal brace across the top of the windshield area. then cross braced the doors and across the front and rear, side to side. all this while the doors are on and fitted so you know whatever you do after this won't affect the door fitment. then you can remove the doors and work on the hinge pillars or firewall or floor, whatever. I always say take lots of pics with dimensions so later you can refer back. if doing the floor area by the hinge pillar (common) you would do yourself a favour by center punching a mark on the hinge pillar, on some good sheet metal above the floor, at a full number dimension like 6 or 8" above the floor. make note of that with a pic. then you can always get the proper floor height later. also, if doing a hinge pillar, make yourself a pattern, from some solid material like plywood or sheet metal, of the pillar outside contour so after cutting and while fitting the new part you will be confident that the door seal will fit proper and the hinge location will be correct. a quick pic showing the dimension between the top and bottom hinge would also be a benefit later.
looks like you will be doing the cowls, hinge pillars, steps, front fender mounts, floor parts and rear cab corners as a minimum. maybe front cab mounts as well. take pics as you go because the hinge pillar parts can be confusing when you go to assemble.
here are few pics so you know what to expect when you start the cowl job. also what I did for a firewall because I didn't wanna fill the 9 million holes perforated through by the previous owners. if your floor is bad in the front cab mount area be sure to keep the proper dimension according to the frame mounts. this will have a bearing on the door fitment as well afterwards.