At the top of the stroke the pistons will actually stick up slightly higher than the top of the deck. This means the head gasket needs to be fairly thick to prevent the pistons from hitting the head. The USDM EJ20's use different pistons that sit flush with the deck and a gasket that is only about half as thick.
Outback Motorsports is one source that sells the Subaru OEM JDM head gaskets for these engines.
Ready for the head.
Torqued to spec.
Lifters and cams installed.
Starting to look a bit more familiar now
Brand new oil and water pumps installed.
Mated and ready for install!