As long as your rear outer longitudinal frames and cross frames are not rusted badly, you should be fine.
Before you cut out the floors, take a good look underneath. Likely you will end up butt welding the rear new floor to the original existing areas where it ramps up. (near the trunk back area) Make sure you leave enough outer edge hanging off so that when you go to grind back off any welds after, you won't be having difficulties getting into hard to reach areas and corners. The original trunk floor edge was tucked under the tail panel and they are spot welded together.