When you fit the rear arches, use the smallest amount of the new panel as possible, putting in the whole wheel arch is pointless, in most cases you will only need the first few inches of the radius, and will cut down massively on bodywork
"Filler" panel is easy, scribe around new panel on top of old, and cut out accurately, you want no more than a 1mm gap to butt weld to, but again, only put in what you need! I would try and stay within the upper and lower swage lines... rust permitting! but it will cut down drastically on filler work.
Thom