[Graham]

Actually not as wide as I thought, 64.1mm across the front and rear arches. 

It's a hefty bodyshell though, 24g with a paper interior.

Amato chassis kit for it.

I would not like to pass judgement until I looked at the overall picture, but it’s not inconceivable. There are some car classes which pass over more than one decade, like IMSA/Le Mans GTP cars which could also be grouped together. But as a principal any cars outside the main group era will have to comply with the same Technical rules as the earlier ones.
I would plan to separate the IMSA ( GTO/GTU) cars from the Gr5 cars into separate Events if I was planning to do this proxy.

But for now the main issue is getting enough support for “Special production cars” proxy for it to be considered, at the moment although we have only 6 responses this one is in the 2nd division. You will need to some serious “lobbying” with your club mates to raise its profile