Box 2 is the body of the car. The instruction book talks about how during the building of an actual 911 GT3 RS, they have a stage where the "drivetrain is connected to the body of the car." This section starts at instruction number 324.
Box 2 contains 9 parts bags which are shown here.
Here's the build at instruction 349. This is the body at the front of the car.
Here is instruction step number 403 with the front together and then the start of the driver's side.
Here's #454. Front and sides are done and working on the back. You can see some of the red pins that are not pressed in all the way throughout the model. While building I was wondering what they were for, but they ended up being the way to attached the body to the drivetrain. There were a few steps where all you had to do was just push in red pins.
#483... almost done with this section.
Here is the final of this section with the body attached to the drive train. The seats and roll bar are in place now.
Just another shot from the front. Okay, enough already... time to get back to building more. Happy building!