screws will protrude, be hard to seal, and invite water, and rust back into your floor and cab. All of what has been mentioned here, follow it. Use sheet metal screws to hold it in place to fit it up. On the flux welder, it does not tolerate dirty metal surfaces. both surfaces, top and bottom, need to be completely clean of rust and oils. Employ a good flap disc on a grinder and get it shiny. The copper welding spoons work great. Harbor Freight sells them for about $10. or flattened copper pipe. Just make sure it is flat. I've used the pipe in a pinch. For a sawbuck, the spoon is just easier. its flat, no creases, wider, and for $10, if it needs to be bent or contorted to reach a spot, then I now have a custom designed tool.
Just give it a spot weld for about a three count, let it cool, move an inch, and spot again. while it is cooling, you can move down the panel and start spot welding in another place. Check the fit after every weld. Remember, metal will move when exposed to heat, especially extremes as in welding. Practice on that old part of the floor to get the welder set right. Make sure your joint is tight. Once you weld, and if you have to put another weld over the top of the previous weld, make sure you put a wire brush on a grinder to clean off the flux. The flux becomes a contaminant if left, and will cause the weld to either fail, or rust.
When its all welded up, and you feel confident that it is right, remove the sheet metal screws and weld those holes shut, grind it all smooth, and hit with epoxy primer.
