If your car is completely assembled with the windshield in, your problem may be getting a 1 piece in the car. I have always found it better to cut the floor out as close to the perimeter as you can then remove the remnants left around the edges, torque box and floor supports. It is always a good idea to support the car in several places to prevent it from sagging when cutting out panels but there is not a good way to brace the car inside where it would need to be without being in the way of replacing the 1 piece floor pan.
If your torque box, frame rails and rockers are sound you won't have to weld any braces inside.
Last edited by det0326; 07-18-2019 at 11:24 AM.