Quarter panel / Pillar help

So....many sins were committed to the back half of this car before I bought it. I am trying to save these quarters. Advice? I am a decent amateur sheet metal fabricator. I don't really care about the hack welding here, the problem is they didn't take the previous panel off.....

Where should I go from here

How flat is that area. Man. I used all metal in my seem and not lead. I would rust prime that area and start my body filler. I used Ice and a dab of bond o in it to stiffen it up so it did not run down the sides of the car. Other's say they use the fiber glass with the short fibers in it. Now it is up to each body guy you talk too. Fill and use your long block to get an even area. Had a problem trying to work that area so put my filler on and dragged a piece of 1/2 inch plastic fuel line across it and that did it for me from the back window to the side window. Learned that trick for a guy putting filler in crack's on a concrete column. While point up the cracks wondered how he was going to smooth out the column. The he reached the top and grabbed a piece of rubber hose and raped it around it and pulled it down the column. Smooth a silk. Good luck.

Is it a flange or is literally a new quarter over the old? Me personally, I start with duraglass over weld seams, followed by regular filler, then a polyester topcoat like USC Icing. After that primer and block.

They left the flange on from the old quarter rather than cutting the spot welds so nothing is lining up that well because of the extra thickness of metal in there.

I’m confused. So they cut the old quarter off under the original welds, then left the strip of OEM quarter on and welded a new quarter to that?

just cut above and below that mess and replace it