The first step to welding in your new piece is to place tack welds in key locations to simply hold the part in place. All seams are butted together, and all welds are a series of tack welds. There is no stitch welding! The goal is to achieve good weld penetration, but to use minimal heat to prevent any distortion of the area involved. The process is simple. Start by placing a tack weld about an inch apart, all along the seam. Cool the area with compressed air, and move back and forth across the part as you go. Then go around the part again, putting another tack weld in between the ones you just did. Do this a third time and you should be left with approximately quarter-inch spaces between all your welds. Resist the temptation to run short bead welds to fill in the remaining gaps. Instead, do a series of three quick tack welds, essentially tack welding the tack welds together. This might sound like it could take a lot of time, but it really progresses faster than you think, and you really save time by reducing your finish bodywork to an absolute minimum. When you have finished, use a 3M reinforced grinding wheel to grind the weld nearly flush with the sheetmetal. Watch that you don't grind any of the body panels as they will get thin quickly. When you are finished, look the seam over closely to see if there are any small gaps or pinholes that might need a quick tack to fill in. A quick pass with an 80-grit grinder is a good idea to smooth everything out.