Rear quarter body help needed

Flanges trap moisture, which creates rust. Look at the panels you're replacing. Where did the rust start? At a flange. Add more flanges, add more places to trap rust. And of course there's a seam. Put the seam on the outside and you have to fill it, put the seam on the inside and you can see the repair. Of the two of course its better to put the seam on the inside, if you have to hide your work under body filler you're doing it wrong.

Butt welding is the harder way to do it, it's easier to warp the panel with heat. It's the correct way to do it though. If it's done properly it should be just as strong as the original panel and there's no seam to detect. The weld has to be hot enough to be a proper weld (you shouldn't be worried about it's strength at all), but not so hot as to warp the crap out of the whole panel. There's a few methods, you can hammer weld as you go, or use a wet rag to cool and shrink the weld area immediately after the weld. The end result is a higher quality repair.