There is lots to tell here. The S600 features very clean lines, and the primary objective was to keep it all clean and subtle; on the other hand the tires extend pretty much entirely outside the original body. So some sort of bodywork was going to be needed.
I took the (expensive) option of buying a second kit. I cut the sides off this second body and attached them to the first body with strips of styrene strip and angle. I added just under 5 mm (4.75" at 1:1) per side. Here are photos of the left side in progress.
The rear section has been widened by about 9" at 1:1, so there is now room for two round tail lamps instead of one. I think this keeps the subtle look better than keeping the narrow rear valance with wider fenders either side.
Love that S2000 driveline! A short piece of driveshaft will be needed.
The rear section went OK because the rear panel is essentially flat; in the front, dealing with two headlights took some planning. The goal is one of those lovely falling curves between hood line and fender line that you see in all the best '50's sports cars. I kept the headlight bucket from the grafted-on panel, and filed away the original, inner bucket. Filling the inner side of the fender, next to the hood line, required some 0.060" sheet for strength, followed by 0.010" sheet and putty for the final shape.
Compared to the unmodified S800, the change is subtle, which is what I wanted. Body width went from 59 mm at the midsection (about 56" at full scale) to 68.4 mm (64.5").
The body seam from the grafted-on panels will be foiled as in the original car; the seam from the inner body, which runs along the door tops, was shaved off. Essentially the outer panel consists of off-the-shelf S600 stampings, with the fenders hogged out to allow the car to sit low over the fat little Yokohama tires.
The body is still a bit narrow and is a long way from covering the tires; the rear tires extend 75.5 mm (71" at 1:1) from side to side, while the fronts are 73 mm (69"). So ideally 1:1 flares of about about 2.5" are needed at the front, 3.25" at the rear. I looked at a couple of options. First was an aftermarket transkit to convert a BMW 2002 or 320i to Group 2 sedan racing trim. The parts, from Scale Productions in Germany, are very well done.
These look good, and, unlike most flares designed for really big tires, fit the 16" rims well. However they are big and wide; a lot of filing would be needed. Also the look no longer features the clean and simple lines of the S600. And given the kit came with an air dam that doesn't fit, I decided to move on to other options and preserve the Group 2 kit for other projects. Second option was to use the extra S600 body panels to build out 'Euro-trash' fenders à la Porsche 935.
This is also a brute force approach, where I wanted the brute force look to be limited to the engine compartment -- subtle these are not. So finally, encouraged by folks on Facebook's The New Modeler's Group, I went for the old classic of a thin lip made of 0.010" styrene sheet, backed with some putty.
The tires still stick out about an inch at scale, especially at the rear; this is fine as it preserves the simple S600 lines and makes it look like the muscle underneath is bulging out of its T-shirt. It's looking very much like a miniature Cobra; this makes sense as I've bumped up the power by a factor of over 4.
So with the majority of the bodywork out of the way, things should move along relatively quickly. Just the hood to decide on... most of the engine will stick out unless I come up with some sort of monster bulge or scoop. Here you can see the air cleaner is well clear of the hood and may even be clear of the fender line.
A paper template gives an initial idea of the amount of hood that may need to be cut away. I may skip the hinges and simply go for four hood pins.
So there it is: an S600 widened by almost a foot. I've never seen one in the metal, but what a tiny little thing the original car was! This is coming along very nicely in my humble opinion. There is lots of sanding and primer in my immediate future...