I did the right rear frame rail on my 83. IMO there is no way to do it right without the body lifted off the frame a good 4-6". Or you could cut a big *ss hole in the trunk floor over it and patch it back up.
In my experience the cutting and welding of the frame rail wasn't the hard or tedious part. It was the body mounts that got me. If you're frame rails are bad enough to need replacing then you can expect to have to do some body mount work as well. I would see how hard the body mount bolts come out(if they do without snapping) to get an idea of how much work you have ahead. If you snap a body mount bolt or break the cage nut free now you gotta go from the top to fix it. All I can say is goodluck.
In my experience the cutting and welding of the frame rail wasn't the hard or tedious part. It was the body mounts that got me. If you're frame rails are bad enough to need replacing then you can expect to have to do some body mount work as well. I would see how hard the body mount bolts come out(if they do without snapping) to get an idea of how much work you have ahead. If you snap a body mount bolt or break the cage nut free now you gotta go from the top to fix it. All I can say is goodluck.