The rear bushing at one time probably could be removed from the control arm but I really doubt that it will be easy to get the arm out of it now on any of them that have a few miles on them
I am with Z spray something on them for a few days to get them free. I think you can get the bolts out if you spray them a few times/days ahead of when you go to take them out. If nothing else the metal is thick enough there to just drill through it all the way into the floor and stick a big bolt and washer on top and put a nut on the bottom to do away with the welded-on nut inside the body. Btw I
wrote a bit more about how to repair the frames here. I added measurements from the shop manual so you can make sure they stay square.
Here are some fancy looking bumpers
