I would not use 1mm for the floor as by the time you have stepped on the section in front of the seats a few times you will end up with a nice bowl shape. I would go with 1.6mm and make it a one piece job with a cut out for the bellhousing as it's the only thing that ties one side of the chassis to the other.
Remember to use stainless 4mm or 5mm rivets with mild steel every 50mm -75mm, if you use ali rivets they will react with the mild when they get wet, and theres no such thing as using too much stickerflex/tigerseal.
If you can stretch to it these will make short work of of it to...
