Arne,
I also have some Shelby Libre style wheels (labled Viper) and I just measured the backspace and is it 3 3/4" on both the ones marked ARE and on the Shelbys. The Shelby's also have a center spine, although it is not as deep as the AREs. The AREs spine is about 1 1/2" deep at the deepest point in the spoke and the Shelbys are about 3/4". The Shelby's, however, do have a cross piece in this section that is perpendicular to the spine and is about 1 1/2" deep.
The ARE pics are first, then the Shelby's. In the pic with the front of the two wheels, the ARE has a weight on it and the Shelby does not.
I figured out the problem with loading the pictures. The resolution on the camera was set too high and didn't meet the height/width limits.