I'd agree, try to repair what's built into the body rather than attempting to cut it out. With all the places you'd have to cut to remove the existing "frame" components you'd probably end up either frustrated and dropping it or with a body that was no longer usable. If it's that bad you should probably look for a better example or if it doesn't matter to you, an Alpine for a rebody of the V8 components.
I replaced the rear legs in mine due to rust, and it was a big job, made easier only by the fact I was going to replace the trunk floor as well.