[strictlypersonl]

The McPherson Strut of the 911 creates an extremely long (theoretical) upper control arm radius. Consequently, the intersection of the horizontal plane of the rack and the line between the lower and "upper" control arm pivots moves around quite a bit. Unless the rack is mounted at the height of the lower control arm, it can get ugly with suspension movement. Note that FWD cars with a high-mounted rack have inner tie-rod pivots close to the centerline of the car.

As far as the 427's spacer, I suspect that they designed it with some adjustability built in, the rack length was less than ideal, or they wanted to build in more roll-understeer for high-speed stability.