It will not warp if you do one spot at a time (MIG), blow air on it until cool to the touch then do another. This will take a long time. If you can't bring yourself to do that, it will warp. If you can, it will not. When you grind, it is JUST AS critical. Small amounts at a time with air or it will warp. It's the only way on large flatish areas on a MIG. You can get some 3/4 inch copper tubing and flatten it. Make some long and short ones and bend to reach in behind and push against back of weld to prevent burn thru. It provides a temporary plug while filling hole. The MIG wire will not stick to copper/brass. If you can get behind, a small metal backer behind hole will weld in place as you fill the hole. Tape it in place if you can reach in, then weld from outside. If it starts to warp, as in: wrinkle and bulge....you are cooked, as in, screwed.
