I have only fitted spacers of almost this thicknes (1 inch) on a 993. As the drive characteristics changed, I removed them.
A thought, Greg: The spacers are made from aluminium. If you look closely on the photograph, at the holes where the hub bolts go, imho there is a serious weakness, there is not much aluminium left.
Risk for failure? 411 are quite heavy cars.
Be aware that the load on the components of the steering and suspension will increase quite a bit. Also be aware that the steering geometry will change, as you are changing the width of the track by 2,5 inches ( 6.35 cm - sounds more in centimeters!
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