It's always a good idea to brace up the cab with square tubing or angle iron before you cut substantial amounts of sheetmetal out.
wooddiver, If you're restoring your truck, you'll probably find that if you're going to replace the entire floor, that in the end it would have been worth it just to buy it instead of making it yourself. If you're rodding the truck, then making it is fine since it doesn't need to look so original.
