In our experiences we have used the stock fel-pro head gasket you can buy at places as cheap as autozone. The gasket combined with a "o-ringed" head has held ballpark 45psi or so on a larger frame turbocharger. We personally use filled o-rings on some of our engines.
I have a Cometic HP head gasket on one of my own cars, & it see's 35+ psi. If you want to hear it from companies that specialize in 4G63's & such, call up Bushur Racing I am sure they have alot of experience. building the badass motors they do..... All I can tell you is from my experiences just like everyone else.

Matt
MFG Racing

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