I think Builder has the right idea! By isolating the systems you can focus on getting the core essentials up-to-snuff! I was fortunate in that my cab mounts were solid....that alone was a major stepping stone. As for floorboard repair, I would really rather have the cab locked into position on the frame and do it one side at a time. If the cowls are bad, as well as the boards you can get into BIG trouble if you don't have everything stabilized. I've seen some door fitment problems that didn't appear until the "repaired" cab was all painted and back on the frame. These kind of problems can sap the joy out of all your efforts.
The 3.55 sounds like a great option....I've never had any experience with a torque tube and had concerns about getting the thing sealed properly so I wasn't dumping the transmission oil all into the diff. Dollars and cents wise...it was more economical (USD) to just eliminate the potential problem completely. Of course, the swap was labor intensive, but I absolutely love to cruise the truck and it was worth the effort and extra down time. I wouldn't hesitate a minute to head out cross-country.
Something folks often overlook about a complete frame-off, is the amount of space needed to store all the various components.
If you stack the sheetmetal you'll always need the part that's on the bottom of the pile and a fully disassembled AD, laid out orderly, will take up the better part of a 3 car garage. And that's before you get everything painted and MUST protect your investment. I sympathsize with those who try it in a 1 car garage. A disaster in the making! Seen too many gouged paint jobs to wish that on anybody!
At 61 (just a kid) you've got some "stuff" left but don't kid yourself....."your final hurrah" will more than likely lead to the "Final, Final Hurrah"!!!!! Been there, done that, doing it again! "Old Dogs" just can't stand to sit around and watch the kids play!!!
Dave
Dave