Stock appearing body panels with a few GARRA-approved changes (airdam, wing, rear bumper) are mandated by Prep 2 rules. Also, a Prep 2 car runs an extremely limited engine package. The TRG/Krohn Pontiac GTO.R's, for instance, actually run a nearly stock LS2.
So far, there have been a few different Prep 2 cars competing in the Rolex series. The very first Prep 2 car was the Goldin Bros. RX8. This was followed by the Pratt & Miller prepared Pontiac GTO.R's.
Of which there are four, two campaigned by TRG/Krohn (championship winning cars, now) and two campaigned by Pacific Coast Motorsports. Also, there were several Prep2 Corvettes, a Pontiac Grand Am from Canada, and the Speedsource/Riley Mazda RX8, and the completely different GTO's of team Horizon.
This year, we're looking forward to the arrival of the HLM Crawford chassis Infiniti G35 whose car was at the Daytona test days this past week.
Prep 1 cars will always be legal, especially as one of GARRA's founding tenets is allowing privateers' equipment to stay competitive for years (hence why the DP's will get chassis/aerodynamic updates next year and Sabre will construct all new DP's, but all current DP's will remain legal until 2012).
As far as the Viper is concerned, the Competition Coupe's bodywork would be totally illegal in GARRA competition (the big tunnelback rear end is 100% dissimilar to the street SRT10 Coupe) and the Oreca GT2 conversion corrects this. In the simplest terms, a GARRA-spec version of the GT2 Viper would be a Prep 1 car, very similar under the skin to a Speed World Challenge VCC, but tuned to use Hoosier slicks instead of Toyo DOT's.