Double-up the old head gaskets, check one bank at a time. My Bud w/a 10sec. Buick GN runs Cometics, plenty of boost, so..., & I put a stock thickness Cometic on Pop's '87 ShelbyZ TurboII 5yrs ago, not one issue.
Just make sure to get the bore closest to Your actual 'top of chamfer' diameter +.020".
