The minimum thread engagement length must be greater than the fastener diameter. I don't know what the diameter of the studs are off hand, but you should be able to take measurements to figure out the largest spacer you can use safely.
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2011 370Z 6MT Sport Gun Metallic | ARC | CJM | Ecutek | FI | Fujimura | R2C | SPL | Stillen | TWM | Z1 | ZSpeed | |