I had the tub on the frame. With the fenders off I jacked up the body one side at a time and replaced the body mounts With the front fenders off, and the rocker panels cut out, if the cab supports and pillar ends are rotten, the windshield frame / firewall will move freely. Putting the top on and checking door clearance before cutting out any metal will get you by. If you are taking the tub off, I think I would weld in temporary supports to hold the fire wall in place if metal needs replaced.
Just my opinion:-)