Hi Tim!
If you would try, I would be very grateful indeed! Le Mans in the mid to late 80s is one of my favourite periods. To make a Group C shape is pushing the limits of what can be done, but we do have a LMP1 shape and Ferrari F40 shape, which if you took elements from each, you'd achieve it.
GP3 is meant to open wheel cars, not closed body. So as will the LMP1 shape and F40 you have to accept the 'oddness' of the rear wheels popping out from time to time. Not too difficult to get used to.
Getting the skin to fit to the shape would be challenging, and a key part of making a shape that is useable, but again the LMP1 car sets would be a good starting point for that.
I will send you a collection of photos and ideas by email today.
Whether you achieve a usable shape or not, I am really pleased you are willing to have a try! Image Group C around the Nordschleife as they did in 1983!
Cheers!
Elio / Philip