Are you going to use the old cut out metal as a template to cut the new metal to size? I am going to be doing the same repair on my car but I am thinking of going all the way up to the fire wall seam and then all the way over to the rocker panel to minimize the butt weld seam. Very interested in your progress on the floor repair.
Last winter I butt welded a spare tire well panel in. If your interested detailed pics are shown in this thread (you'll have to scroll down):