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I'm not sure how much clearance you need but I used these to gain some extra room for my roller rockers with stock covers.
Of course this was on a small block and I was using World Products heads that have a raised valve cover rail, but the gaskets are about .3 or .4 inches thick. So that is thicker than a conventional gasket, they don't crush and they are reusable. Earl's perforance products makes them and they are available thru Jeg's and Summit. p/n EAR-29E04B for a big block.