Hips. Now THERE's a thread.
Actually, Paolo, RCR offers the wide bodywork I believe you are referring to; I'm pretty sure Bill D would likely confirm that for us.
Thinking back, I seem to recall the better part of a conversation quite a few years ago with Tom Armstrong, owner of 1033. When discussing this n that about the car, somehow the topic of bodywork came up. Tom commented that he'd recently had to replace a few panels on his GT and -this point I found very interesting- during the panel process he'd learned that the rear clip came in three different sizes.
I would imagine that 'production' cars in those days were required to have full body work and, as you mentioned, competition tires were just getting wider and wider, so right along came that 'broader' rear clip. I wonder how much of a factor the wind tunnel was...
Best,
T.