I would remove your entire interior and anything that you don't want ruined by weld spatter. The tinyest piece of spatter will stick even to glass. From below, remove anything you're able to remove easily or are going to replace anyway (drive shaft, brake & fuel lines). Also note that the floor holds the rear subframe mounts in so you will need to block both the body and subframe up. You may need to remove the steering wheel to turn the floor at the angle you need to insert from above, otherwise it shouldn't be a problem.

We did mine on a rotisserie. The sides and H brace get welded in from below so be prepared for a lot of welding on your back. The full floor is a very nice piece IMO.