Be careful when you do, it's pretty easy to warp the raingutter since it's thin, and you'll just be welding on one side of it.Screw up was drilling the thru holes in the pillar and not just the plate. I’ll have to close pillar holes up before welding plate back on.
You can access that cavity from below, through the kickpanel. The dome light circuit comes down the DS a-pillar, and it's open all the way through. There's one seam that's an obstacle, but that's it.You might want to consider spraying some Eastwood paint inside the pillar while you have the holes. Use the wand that comes with the frame paint.