Darien, you are pretty close. From where you are now I and removed the nut part just leaving the "cage" hanging there. Then I removed the kick panel so I could access it from the back side (removed the same size as the new patch). now once you have measured a thousand times and drilled the holes in the new pillar and removed paint and prepped both surfaces to weld, You also need to remove paint where the cage nut will be welded to the new A pillar.
Then I put a couple tack welds to hold the pillar on but not on the front surface where the hinge nut is, put the welds on the outside edges away from the hinge nut area. Then This is where I used a piece of aluminum stock cut to fit inside the pillar but you could use a copper welding spoon or any other weld backing tool that will fit, clamped in place. The purpose of this is so the weld does not protrude through the inside of the pillar leaving an irregular surface for you to grind. Having your weld come through may interfere with the hinge nut being able to move around for adjustment/alignment of the door. That my theory anyway.
Next I would weld the pillar fully from the outside, remove the weld backing tool, grind and sand out the weld. now put the nut in the cage and put a bolt in one of the holes so you don't drop it. clamp the cage against the A pillar (gently its fragile!) and weld the bottom tab of the cage (where you drilled out the spot weld) to the pillar.
You may now proceed to fit (fight) and install the kick panel.
PS- Cats are evil
