Agree, plus maybe the gasket makers are also leaving some extra margin for engines with larger than stock bore size due to overbore during rebuilding. Stock bore size is 76.5mm and the second oversize goes out to 77.5mm. I don't think I have ever seen a special overbore headgasket so they probably manufacture them all to accommodate the largest possible bore diameter.
The 78.5mm size of the gasket would do that plus a little room for error with gasket positioning as you say to be sure it will clear the protruding pistons. So that 78.5mm size is probably appropriate.
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86 745 D24T/ZF 345k lifted 2.5"
83 764 D24T/M46 155k
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