The key is constant heat to create constant warp with TIG welding. You can hammer and dolly to stretch the weld area back out to remove the warpage. Then 80 grit disc on a die grinder and 89 grit DA. Also, I can't stress enough, metal fitment. Minimal gaps for TIG equals no filler rod needed or at least minimal and keep the surface flush so when you gring the weld area down it dissappears. TIG welding sheet metal is nice because the weld is so small or nonexistent.
Now, I mig welded the bottom plate in because it was a long weld and out in the open, meaning no support from and curve, bend or body line. That leads to easy warpage so I wanted to put minimal heat into the pane. With mig I just do a billion really hot spot welds but then hit them with air to cool them really fast. Now if mig welding you want a tiny gap between panels because as the weld cools it pull/shrinks and can cause fitment issues but you still want them level with each other so the wleds will dissappear. The key here is really hot spot welds so they almost melt flush and you don't have a huge lump if wled to grind down. And to cool them really fast so the heat doesn't warp the panel. Also jump around again, to keep the heat down.
Mig welding creates a hard weld that can't we bent easily without cracking, TIG welds and be manipulated pretty much just like the surrounding sheet metal so you can hammer and dolly them, bend, stretch and shrink them.
