The stock lugs hold on the spacers. They are M12 x 1.5mm thread pitch.
I don't remember the length, if that is what you are looking for.
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LTZ 17s
Eibachs
20mm Adaptec spacers
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LTZ 17s
Eibachs
20mm Adaptec spacers
Tires I'm not so sure, but 18x8 +30-35 would be a good start I'd think. If you look at beastlicks photo above as a reference, a +35 would be about 1mm further in and a 30 would be 4mm more out.
I have 17X8 with a 40mm offset (ASA GT06's). I am running a 5mm spacer too
here's the size of a 17X8 compared to the normal width of the LTZ 5-spoke tires/wheels
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Here's after with the spacers...no rubbing or anything, not HELLA flush, but more than stock by far!
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If you go to an 18" you'll want to do a +1 conversion (same size overall diameter). 8" is roughly 204mm, I'm currently running 225/45/17