I've done this a couple times. On a '69 Charger 500, I replaced all of the floors from the firewall to the tail panel.... on a '69 GTX and a '70 Superbird, I did the same thing with the exception of the rear seat section area. The best advice that I can give is... just take your cutting out the old metal. It is easy to get in a hurry and cut something out that you did not intend to. Use a wire wheel to clean up the original spot welds so that you can clearly see where they all are. To remove/install the rear seat floor panel, you will also need to remove the large V-shaped brace assembly that is behind the seat frame. Take special care not to distort the brace so that you can reinstall it later. Another thing that will help, go ahead and weld the spare tire brace, the fuel tank brace, and the fuel tank strap braces to the the trunk floor before you weld the floor in place. It is a lot easier to do this out of the car than to try and crawl inside the trunk and weld. Hope this helps a little.
Justin
"CornDog"