Big Dave, I did/am doing what you are planning. I replaced the floor and trunk pans at the same time. The tail pan is also out. The only bracing I did was from front to back on each side with a 1' X 1" angle from wheel well to kick panel brace, and a piece of same tying the two together. My only concern was that when I cut the floor out, the roof might buckle - either way. I didn't want to brace it rigid and find out the pans were off 1/8" and couldn't 'massage' it. I didn't weld it in yet, as I'm replacing the rockers and toe boards now. The original rockers were long gone, and the covers over them were shot too. I'm using 56-7 rockers since they don't make 55 4 door rockers. It's turning out to be quite a project!
If you click on the link in my sig, you can see a couple pics of what I did. Good luck!