The Corvettes were given an 0.4mm wider air restrictor, a richer fuel mixture, 2 liters more fuel, and can shed 22 pounds.
The Ford GTs adjustments were to add 33 pounds, and turn down the turbo boost (2000 - 7200 rpm).
Some of the other competitors had changes also.
Apparently, rules can make or break winners. If only there was a series where "production spec" GT cars could race against each other.