This is interesting... I missed this the last time I was there but found it this time.
It seems BOTH tube framed Corvette bodied cars AND PrattMiller C5-Rs are approved for Grand Am GTS!
Very interesting...
Excerpts from the official grand am site follow...
http://www.grandamerican.com/competi..._GTSrules.html
1-2 GTS-approved Production Based (Unibody) Cars – Normally aspirated engines - maximum displacement 366 cubic inches. Approved *Turbocharged engines - maximum displacement listed below.
1-2.1 Corvette – Pratt & Miller C5-R
...
1-3 GTS-approved Tube-Frame Cars 1998 to 2003 body styles
1-3.1 Chevrolet - Corvette, Camaro and Monte Carlo