OUCH!! Not trying to be a downer here, but I would have to say ... I no longer try to cure that kind of cancer. My first car was a '67 VW beetle that had cancer almost as bad as yours. I spent countless hours grinding away the rust and repairing it, only for the rust to show up again several years later. I learned my lesson from that, and whenever I buy a car now, I crawl all over it looking for rust.
I've had my XJ for more than 12 years, and plan to keep it forever, so I just wouldn't want to start with such a massive amount of cancer. You also need to consider that if you weld in entire replacement panels, the unibody "frame" might not be in a completely aligned position during welding, which would create tracking/alignment problems.
Sorry about that, but I agree with old_man. I'd look for a decent body and combine the two vehicles.