If the cage is welded to the frame that is preferred for safety. However, if you’re not going to wheel it and its more for looks it opens some doors for possibilities!!
Three options I see here:
1 - Find someone parting one out, buy the tub, and cut the pieces off you need and weld them onto your tub. Not sure how experienced you are with a welder/cutoff wheel/shaping bondo but it would be a good way to learn.
2 - tub swap. Cut the cage off at the frame as close to the frame as possible. Remove the cage, swap bodies, and try and re-weld the cage to the frame. If you decide to go this route I learned some tricks after swapping in a cage in my current K5
3- Still a tub swap. Cut the cage off at the frame, swap tubs, then instead of welding to the frame, cut the legs of the cage down to fit inside the new cab. The idea here would be to weld the shortened legs of the cage to some plates and bolt the plates to the body. It’s not as safe as going to the frame but its better than nothing. Plus you have the cage already.
I guess the question would be, is the cage you currently have built well enough for your needs? Is it worth the effort to you to make it work and fix the rust?