I use 25mm in the front and 22mm in the back. You can also put 20mm in the rear if you want. You don't need to shorten or modify the studs with these spacers. They bolt right onto the hubs. I had them on my SS for a few months before I put my new wheels on. They look great and give the car a more aggressive stance. I'm selling mine in case anyone wants them. They're in perfect shape.
Here're a few pics with the spacers installed:
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