You don't mig or tig sheet metal they would blow right thru it turned all the way downWhen the shop gave you the estimate of 1200 dollars, that was probably for purchasing a new core support, having it painted, welded in place, and putting your car up on the chief to have the frame aligned in the front. If you just slap a core support on there and bolt everything back together you might not like the outcome when your bumper, hood, and both fenders don't sit right because you were an eighth of an inch off on your measurments
Edit: Mig or Tig would work