Reaction Research has several body kits for the first gen Z car: SubtleZ. 280YZ, a 944ish kit, ect. All have full fiberglass front fenders, airdam and hood options as well as fiberglass rear over-fenders.
http://www.reactionresearch.com/automotivedetails.htm
As far as a smoothed out rear valance, that's a custom deal. The 280Z doesn't have the bumper indents on the rear fenders like the 240Z does, so it's easier to smooth out. Remove the bumper, weld up the holes and smooth it out with filler.