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NFSBOSS said:Any spacer or thick washer should work but you don't really need it regardless.
I don't think there's a real answer to your second question and the race cars to 8,200+ on springs. I wouldn't go beyond 7,900 on stock springs. Ask Shaun at AED what he thinks.
My point is that installing different cams and valves/springs and make additional HP so if that's what you're after include those upgrades.