If I remember correctly the two GNs in this video were owned Dave Bamford (top gun) and Jeff Fox... I don't know what mods Dave's car had at the time of the race, but here's what Jeff Fox's car had....
If someone will provide me some FTP upload space I've got it, as well as a copy of American Muscle about the GN/GNX. For some reason the latter is in french though, so you sort that out.