Do the spots and tacks, then when you are welding use a stitch method. Any sort of continuous weld will warp the panels.... I usually do a stitch on each tack, then lay the welder down for 10 minutes or so, then repeat the process. Makes for a long project, but beats the heck out of warping everything. They used to sell a product called "Moistbestos", but as the name suggests it had asbestos in it and has been banned. You surrounded the area to weld with it, and it absorbed a lot of the heat and prevented it from distorting the panel. I really prefer a tig for repairs, but if you don't have one you gotta make do by adjusting your method of welding.