Floor pan removal and replacement?

If replacing the entire floor pan at once, I suggest adding some cross bracing between the sides of the passenger area before you cut to be sure you hold the factory dimensions. Depending on how the vehicle is supported, and whether or not the engine / tranny/ rear end is still in place, you can get a lot of flex if you start cutting without some reinforcement to hold it. If you watch this type of work online or TV, you can see how the bracing is done. And double what Syleng1 said about welding practice. I had 20 years of experience as a welding supervisor in shipbuilding and welding on thin material is an art that takes practice. Looks easy, but it's easy to screw up.