Zilch, based on comments attributed to Duntov, the ZL-1 Corvette production number of 2 is probably bogus. That's how many were sold, not how many were built.
As a neutral observer, I looked at the rules on http://www.geocities.com/psmcdr/rules.html. They seem to meet the spirit of factory stock. To be honest, 11.54 for a heavy N/A non-nitrous L-88 Corvette is impressive by any measure, let alone with the tire limitation.