When we stripped down our car we didnt use bracing between the doors. A totaly stripped down conv. unibody only weighs about 400 lbs. We were careful to keep a 4x4 support under the cowling and the rear frame at all times and if we needed to get in the body we added another under the trans tunnel/rockers. We didnt have to cut or replace any flooring or rockers though. If your going to be cutting large sections of the floor and replacing the rockers, by all means add braces to the door openings like Salmon38 mentions. They can be welded in place and ground off later.