It's really easy. Heads for 1997-1998 DOHC turbo are as follows:
EJ20G: shim over bucket, normal valves and springs, all same as USDM EJ25D except for the cams
EJ20K: shim over bucket (same as EJ20G), specific EJ20K valves and springs, specific EJ20K cams
EJ20K STI: shim under bucket (unique for EJ20K STI and 22B), same valves as EJ20K, specific EJ20K STI springs, retainers, same cams as stock EJ20K
The castings for the above heads are nearly identical, so if you want to convert to aftermarket setup use USDM EJ25D setup. If you want to use shimless buckets use the 2004-2008 EZ30R exhaust buckets they are 33.0 mm in diameter, same as the 1997-1998 EJ20G and EJ20K buckets.