I think that this is what you mean? (see pics)
I was going to swap tail panels, but changed my mind. By the time you work one over then do the swap, we'll all be dead. I had some pinholes in my original tail panel near the lights. I simply mig'd the pinholes, deliberatly burning out the thin metal. See the third pic and spot where the pinholes were filled. Larger spots required small patches. The license plate area, because it is a second piece spot welded to the tail panel (and surrounded by body seam caulk) will also likely have rust. Pics one and two show what you might be facing...and no, you won't necessarily see all of the rot without taking the pieces apart. The second pic shows a new piece that I fabbed up and welded in to replace the original metal.
As far as where the quarters meet the tail panel and the trunk gutters, you can drill the spot welds, cut out the bad metal, and fill. I've also attached a pic of some of the trunk gutter work I've done. Fortunately, I didn't have to get into the top area on the tail panel.
Just work the panel as best you can. Weld in new metal to replace rotted sections. I would personally avoid lead. There's a reason that went out years ago. There are many quality filler products available today that are non-porous and can be applied to bare metal. Whatever route you go, make sure to grind, cut, burn, file, or otherwise remove every bit of rust that you can, remebering that the whole car is put together with spot welds, meaning that there are a lot of panels laying on top of panels with only 33 year old body seem filler (which is now brittle and cracked) to keep moisture from getting in between the panels and totally raising hell.
Just my $.02, which is probably worth less than that.
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