You may have heard about hammer welding before and marveled over it, but here's a quick lesson. Like I mentioned before, weld about an inch using the MIG wire and very quickly set down the torch and the MIG wire, flip up your helmet, and grab a body hammer and a dolly. With the dolly directly under the weld, hammer along the weld while moving your dolly to stay under the hammer's blow before the weld cools. This will stretch out the weld and the sheetmetal around it back to its original shape. If you leave the weld to cool on its own, it will shrink, which causes the area to warp. | 
I tried to cut the edge of the wheelwell as straight as possible, because once the straight sheared edge of the widening piece is butted up against it, you only have to weld up a very minimal gap. |
When welding a tight gap in sheetmetal, you can use 0.035-inch MIG wire as filler rod to tidy up the job even further, because 1/16-inch rod can often be too much filler. Either way, I still only weld about an inch at a time to avoid warpage. Even though my Miller Dynasty 200 TIG has lots of amperage for welding thick stuff, when dialed down to about 47 amps and paired with the MIG wire, it does a beautiful job on this 18-gauge sheetmetal. | 
It's tricky to get used to and good at, but the results are very pleasing once you do. Here you can see how small and flat the weld is now and how all the separate little welds have been joined. Almost no grinding needs to be done afterward, but after welding any thin metal, a little more hammer and dolly work is usually necessary to ensure straightness. |
Since I'm happy with how the new wheelwells turned out after widening them, I can start removing the old ones. I drilled out the factory spot welds for the stock rear wheelwells first with a small 1/8-inch bit, followed by a 5/16-inch bit. | 
I marked the width of the new and wider wheelwells on the bed floor to see how much of it I need to cut out. |
Again, straight cuts will help the job turn out nicer in the end. You can see that I have yet to fill the hole in the floor from the step notch in the frame. | 
To finish off, most of the factory flange on the wheelwell was welded back to the stock panels underneath like it was when the bed came off the assembly line. When people see the inside of the bed when the truck is painted, hopefully they won't even know I did anything to the wheelwells! |