don't bother... cut it off... I've done em both ways, and cutting that off is a little easier..
look at the truck, leave a 3/4" of the qrter hanging below the seam (as a weld lip), then cut straight across the bedrail where the "post" turns, when you get to the back of that post tab on the new qrter, leave the little "up" flange (drilling the spotwelds out is best, then you plugweld it back to the post on the truck)
obviously its best to cut the new qrter after final trim of the truck..
The other way is what your guessing, it's all doublewall panel... you need to run a LONG flat air hammer bit up from behind to get the upper spotwelds on that tab... the easiest way to do this is to relief cut the outer sheetmetal at the corner and peel it back some... weld up the relief cuts after reassembley...
I did it that way on my current rig, but it is a little more work...