Hi Robbocop,
That link doesn't work. Do you mean at the bottom of this page?
http://www.mcracingsa.com/RENAULT5ALPINE.htm
Now, by all accounts, that wheelarch looks crescent shaped as you said. It will require modification to the rear arches. Many of the kits will - incl. the Calberson, etc.
I don't have any experience of fitting bodykits, but I do understand how an R5 goes together!
You can see from the photo that the arch cover extends up into the rear quarter panel much higher than on a standard car. I would think you'll need to mark out the panel with the arch tacked in place, and then cut (somehow) and rejoin the metal. Don't ask me how to do it "properly" and not be a bodge!
When you've done that, you should be able to fit wider wheels, but make sure they're wider only in the outer dimension, 'cause if they're wider towards the middle of the car they'll rub on the arches that side. You may have to modify the inner wheel house anyway.
I don't know the specs for maximum permitted offsets, etc. can anyone else advise?
Good luck with the project.
Andrew