I'm about to get started on a similar project on my '51 too so I guess we can cry on each other's shoulder
Remember that sheetmetal can move around quite a bit so don't go cutting out everything and expecting the cab to stay "plumb". My plan is to do a piece at a time and do as much as I can with the cab in place... the same place it needs to go when it's finished... kind of using the frame as a fixture. Once you pull that box off the frame you're going on faith... not a good idea. Many an extensive panel replacement job has come back to haunt the owner after paint when it came time to hang doors.
Another trick I learned many years ago is to dry fit as much as practical and then tack the thing together. When we used to MIG weld panels we just wore a heavy glove on the left hand, pointed the wire where we wanted the tack, cupped our gloved hand over the welding tip and squeezed off a BB sized tack. Then go to the farthest distance from that tack and did another and then put one half way between each until we had a tack every inch apart. Many times we just kept tacking until the weld was solid with no gaps. But in the 70s sheetmetal was a lot thinner and more prone to warping so that method may not be practical. If you've never welded sheetmetal before you can get yourself in a big mess real fast. Measure, measure, dry fit, dry fit, tack, measure, check... As tedious as it sounds it's a lot easier than trying to fix a warped panel or even worse a warped cab.
THIS is what I'm up against. A PO with the soul of a butcher did this to mine.