If you want to be able to open the doors, you are going to need a spectacular amount of reinforcement in the rockers to stop the car from flexing there. The roof and A and C pillars form a truss assembly which reinforces the front and back sections of the body. Suspension loading is going to result in increased body flex. You also have to deal with the fact that you have likely lost most of your rollover crush resistance. It will be extremely difficult to adequately reinforce the A pillars and the top of the windshield. A pillar reinforcement and that top member reinforcement may be a must even if you are not concerned about roll over. Increased body flex may crack your windshield if you don't reinforce those parts.

Doing a quality job is a daunting exercise. If you want to go half-ass, weld the doors shut. That would provide a lot or rigidity to the tub.