Bill
Assuming that the non dubious parts are still in alignment you should indeed replace the bad stuff before removing the body. Use the fit of the doors as an indicator bearing in mind that MGA doors had a habit of warping outward at the bottom rear as a result of their aluminum construction and the constant slamming the recieved against the often too hard buffers! Doors off, doors on, a million times. Try to be patient. It will pay off. When you are happy with all the other stuff you are ready to remove the body but STOP!
If you really don't want the body to buckle you must build brace fore and aft at the tops of the doors and diaganally across the cockpit as well. Only when you are convinced that the whole body forms a rigid unit should you unfasten the body and lift it. Good luck.
Motorbill
From Lola to Land Rover, If it's British and has wheels, it's likely I've bloodied me knuckles thereupon