The big question you need to ask yourself is, what kind of truck do you want when you're done? A frame swap almost inevitably turns into a frame off restoration.
Patching the whole truck up "good enough" to where you can drive around in it, or doing a complete tear down and rebuild are both viable options. The time, money, and resource inputs are far different, as are the final outcome.
No shop competent enough to repair/rebuild sections of frame will be cheap. The amount of money invested in the project can become a bone of contention with loved ones.
Personally, I would make it run and drive. If the drivetrain felt good, I would just build a new section of back frame from rectangular tubing, close to original dimensions, but not an exact copy, and put a flatbed on it. Then patch the body good enough to pass inspection. But I'm a little cooky. The big question is, what do you want?