I wouldn't weld it...especially if this kind of work isn't something you've got a comfort level for. You can find yourself in deep water quickly welding this thickness of metal. And welding is something I learned nearly 60 years ago and have done commercially. This is a non-load bearing, non-stress area so I'd first get my replacement piece in hand, and then I'd carefully cut out a rectangle in the dash using a saber saw and a thin metal blade. Go slowly with good light. Then make a pattern on a piece of thin cardboard (like cereal box). Hold that thin cardboard under your dash or tape it on so it fits all around the curved area and using a sharp pencil trace the cutout shape onto it. Make careful note of where any knob holes ought to be. Then, cut out your cardboard model and transfer that to the replacement piece carefully aligning for knob holes, etc. Mark it on the replacement piece carefully and cut that replacement piece to fit. Then I'd do two things: 1. epoxy a metal flange (same thickness of the dashboard metal or slightly thinner) around the perimeter of the opening on the back side of the dash 2. bed the replacement piece in on top of the flange you made once it has cured with either JB-Weld or 3M Panel Bonding epoxy. Sand both the flange and the replacement piece well where it will be epoxied. Both of those products are going to work about the same, by the way. Finish with a thin coat of body filler, sand and you'll be done and ready to prime/paint. PS...if you're concerned about using JB-Weld, please don't be. Below you can see an area where the roof pan meets the upright pillar on my truck. Around 1978 this seam cracked and I repaired it using JB-Weld rather than weld it. There's no line at all even after 40+ years. Good luck.