Update 9/25/13:

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The builds on all of the GT cars were revised to balance all cars at 567 PI. Rear Aero is no longer mandatory (Front Aero not permitted). If you choose to run rear aero, you are free to adjust the build as long as the car stays at 567 PI. This excludes the Jaguar, the DB5, and the Porsche.

The builds on the prototypes were updated as well. They are all now 652 PI. Aero on the prototypes is not optional - the Alfa 33 Stradale is required to have areo on the rear, the Ferrari 330 P4 must run with no aero, and the GT40 has non-removable rear aero standard. Rear aero is optional

Update 10/22/13:

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GT cars now have the following rim options:
American Racing Torq Thrust M
Cragar S/S Super Sport
Halibrand Sprint II

Rim Diameter is not to be changed
It is prefered that the rims not be painted, unless photo proof is provided indicating period accuracy.

Other rim choices may be taken under advisement

Update 10/29/13:

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For Nurburgring Nordschleife ONLY - full adjustable aero is permitted, with the following exceptions:
Jaguar E-Type may not run Forza Rear Aero. Blame Turn 10 for the God-awful wing they gave that car
Porsche Carrera