I agree on a combination of suggestions already mentioned. I did this repair on mine and when finished right, it's invisible and looks stock.
1) Drill out the spot welds that hold the retainer and flat nut in place on the inside of the jamb - if it's still there. (You'll have to remove the interior panel so you have access).
2) Cut a square hole in the jamb with a cutoff wheel, removing just the mess that's there. From here on out there's no one way to do it. This is just one way.
3) Cut a square of thick sheetmetal larger than your hole. Use a few sheet metal screws from the outside around the hole to hold this piece in place on the inside of the quarter. You're going to plug weld these screw holes shut later. This will be easier than trying to get a welder inside the quarter later.
4) Now, cut another piece of sheetmetal the same size as the hole you cut. Trim it to fit flat and flush - with maybe a 1/32" to 1/16" gap around the perimeter for the weld to penetrate (butt weld). The larger reinforcing piece you put in earlier will keep the patch from falling through into the quarter and hold it flush. You can use a couple of sheetmetal screws to hold it to the backing piece if you want.
5) Weld the perimeter of your patch. Begin removing sheet metal screws and plug weld the holes. This will join the layers together.
6) Take measurements drill a hole through both layers for your striker. A step drill works great.
7) Position the nut retainer so the nut can "float" within the hole you drilled. Tack weld the retainer in place. You could also drill small holes through all 3 layers and plug it.
8) Grind your welds smooth and finish with filler as necessary.
