Here is an easy way of cutting out spot welds, as long as you are not trying to save the top sheet. Using a 20,000 rpm cut-off air grinder with a 3-inch wheel, I grind a spot weld through the first layer of sheet metal, and very little into the second sheet. You can get good at this pretty quick, taking 10-15 seconds to disconnect each spot. With this narrow strip being removed most of it pulled up easily by hand, with just a couple of spots wanting a little encouragement from a screw driver or another touch with the grinder.
There's the outer strip after removal. I was pleased to find it was not welded under the top body cowling. After removing the accessible spot welds the strip pulled right out. That hole that was hidden at the top end appears to be an access hole for a spot weld electrode for welding the top cowling in production, as the B-pillar was a subassembly prior to installation on the sidewall but before the top cowling was installed. Notice in the center photo above there is a U-shaped brace/bracket between the striker plate mount in front and the back plate at location of the rear gusset. That part is in good condition and will be a keeper.
Now I knew the pillar rear plate was perforated at top and bottom. Good news is it appears to be solid in the middle between the rear gussets, so maybe all it needs here is a small patch panel at top and bottom in back. There is a rash assumption here that I will be able to get inside with a sand blaster to clean out the surface rust well enough for inside paint and/or seal.
More good news is the front plate seems to be solid all the way up, including the flanges. If I can clean the rust out well enough for sealing, the entire front plate should stay. This job just got easier. Photo at right above shows a thick steel plate I welded in some years earlier as a repair for the top frame mounting threads. Still in good condition, that's a keeper too. (15 Dec 07)