Lower Quarter to tail corner stack up

[iwlbcnu]

Putting on lower quarters, and confused as to the stack up. The lower quarter welds to the tail corner, but there is a third layer from the corner post that only had a spot weld where it all met the rear floor cross brace. From there up was not attached, and looking at other bodies, it may have only had some sealer between it and the lower quarter. Why would they have a panel just hanging out?
Now it is seeming like the best way to install will be to weld the corner to the lower, then set it all in place and finish up since you can't really get into the light opening to do much.

 

[U13FL735]

I had the same thoughts when I got into replacing mine....why have the two layers on top of each other, but not welded together? My guess is because the lower quarter and the extension were put together first, then attached to the upper quarter, as you're thinking of doing. I welded mine together one piece at a time (after alot of fitting up and adjusting, of course) and think that I had better control of the overall fit, and ended up with better gaps at the doors, and tailgate. You can work thru the taillamp opening and get a half dozen or more plug welds to attach the extension to the quarter, but there's not alot of room to get in and finish up those welds. I'm not saying mine was the right way, just the way I ended up doing it. Why not mock up the two pieces together with clamps, or screws, and see how hard or easy it is to install in one chunk?