I've never in my life had a header bolt come loose unless I used a thick gasket, and I would never use a thick gasket with a turbo.
If a bolt comes loose or comes out, something is wrong with the hardware stack or bolt. Thick soft gaskets can yield and let bolts fall out, but they blow out anyway.
I mount my turbos right on the header and use normal bolts. No gasket. Just high temp RTV.
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89 LX. 363, single turbo, Super Vic EFI, TFS high port heads by TEA, solid roller, glide. Holley HP EFI. (exact combo varies)