Felpro head gasket 521SD with nitrous

If it's got RPMs on it, IMO you've already got the wrong gasket on it. I'd pull them and replace it with the 1008s. The rounded fire ring on the 521SD will not compress as well with the aluminum. Over time it will lead to brinneling and wear a groove in the head. But short term it may have less sealing ability too. The 1008s are pre-flattened.