There are some killer places for bonding adhesives in customizing, that is for sure. If you do, bevel the edge of the metal,that will lesson the chance of a "ghost line".
If you are welding a hole, filling the hole with a piece of metal and then butt welding the seam, you can control it as you weld and end up with little to no warpage. But if you were to weld a piece OVER a recessed area like with a 1990ish Toyota pickup, you REALLY, REALLY have to go slow.
I have done that a few times, do you know how long I take to weld it? How about a DAY, that's right, I would take a day to weld it up. Two or three small tacks and then let it cool completely down. I mean cool enough to put your tongue on it and feel no heat. If you weld it that slow, you will be successful.