I finally bought some 18 gauge sheet metal and started to put the floor back together again. I discovered that I suck at welding thin sheet metal lol. But, after a couple of tries I was able to stick it. I had to cut the sheet metal and form it into an angle and weld it to the back section of the cab. I needed something to attach the new floor pans to to keep that back panel in place. I'm glad those welds will get covered up lol.
Practice, practice, practice. I use a Miller 175 110 volt MIG, and run .035 solid wire, and 75/25 Argon gas. The .035 wire does real well in filling holes and controlling heat. The biggest thing to work on is heat control by focusing most of the puddle on the strongest metal. Even the rusty original metal takes heat better then the new metal that the floor pieces are made of. Modern metal just doesn't weld as nice as the original. The next thing to work on in your wire speed and amperage rates. I will burn at a higher amperage, then adjust my wire speed to get the desired puddle. From what I can see, you did fairly well. You can take a body hammer and dolly and flatten out any raised weld. This will also help to keep the weld strength. Often times one will be too eager to grind down a weld to make it pretty, but will remove too much material and make it weak. If you are planning to butt weld the new metal with the existing floor, remember to run and gun. Spot weld in a pattern that will reduce the amount of heat drawn in one spot for too long. This will cause your metal to ripple and pull. Included is a picture of when I was not following my own advice and was trying to hurry too much. The picture is of a passenger floor board replacement to where I didn't move enough and the corner had pulled upward into the tool box. To fix it, I had to heat the section with a torch and pound it back out flat...from inside of the tool box.
It is a bit of an acquired talent. Coming from a heavy equipment background, it took me a bit to figure out how not to make a panel look like the ocean.