I have built the rear bracket for the c4 IRS from images I found online and stock c4 diagrams.
The rear is quite a bit wider, I have custom wheels that I had cut for this setup but will still have some work to do on the width of the body. I don't mind flares but also don't want to ruin the lines of the car. So I have to take it slow.
If I had that rendering back already...
I will post more updates when I can, I will be replacing the rear frame rails with a 2x3 frame and am currently considering a full frame under the car. I found rust in the rear portions of the rocker supports the other day.
I really wanted a deep dish wheel on the back but could only fit so much into the narrow body.