For the floor panels you can get that Flanger tool which lets you overlap the metal and weld it and it will still be level.. It just steps the metal down basically and leaves a little step to let the other metal sit on top and weld. Get what I mean? Now if you do patch panels on the body then when you cut out the metal just cut out the rust and then cut a new piece a tad bit smaller than the hole because when you weld the panel in it will all expand together and fit nice, etc.