Hello Paul - me again!
An Australian contact sent me a picture of his car, and questioned my post, as his car tended to appear like your 'red' photo. So I went to the garage with tape measure.
On both the S1 and the S2 1600, the distance from the swage line down to the wing bottom, in front of the wheel arch is 13" (sorry it's imperial - it's an age thing). The distance from the swage line down to the bottom corner of the wing behind the wheel arch is also 13" (give or take .25"); and in both cases, the rear wing bottom profile sweeps upwards from the lowest point at the end of the arch.
However as has been correctly pointed out, no panels are interchangeable between S1 and S2, and panel shape can vary from car to car, the factory using lead or filler to get the shape/gaps right.
I would suggest that as Bill has made a beautiful rear valance for you, that you just blend the lower curve of the wing in to match the edge of the valance. Remember that this now creates an in accessible chamber under the boot-floor inside the valance and rear wings, and is a candidate for injecting cavity wax or similar to prevent corrosion. I also sealed round the edge of the boot-floor/inner face of wing/rear panel with Sikaflex to prevent any moisture getting from above. Remember also to have fitted in the hatch drain pipe hoses first.
Regards - Roddy