I personally welded in cross-braces when I cut out my floor pans just to make sure it didn't shift at all. And I only did one side at a time. So far, no twisting, but I still don't have it back down on wheels. Considering that it's a unibody car, I'm not sure I'd cut out BOTH floorpans, and their connected frame rails without some sort of cross bracing to make sure nothing gets tweaked. Especially since you're never sure what other rust is lurking in places like your rocker panels.
Of course, I also welded on both the replace ment rails that came with the floor pans, AND the bad-dog rails over top of them when I replaced my floors. I tend to over-engineer things however.