Fred Gibson had some great ones, here is one article about them, I've got the magazine at home, I've read it so many times, I love these stories. My favourites would have to be the Oil pressure guage/boost control and the 'standard intercooler in factory packaging', fire extinguisher one is good too..
http://wiki.r31skylineclub.com/images/d/dd/Gibson1.jpg
http://wiki.r31skylineclub.com/images/7/77/Gibson2.jpg
http://wiki.r31skylineclub.com/images/6/68/Gibson3.jpg
http://wiki.r31skylineclub.com/images/3/37/Gibson4.jpg
http://wiki.r31skylineclub.com/images/a/aa/Gibson5.jpg
http://wiki.r31skylineclub.com/images/7/75/Gibson6.jpg
Not a GrpA/C one, but Bill Elliott's 7/8 scale Nascar is a legendary cheat.