So, you know all the fundamentals from your past exp but your goal of a Group A GTR as a benchmark / standard is abit dated.
From my exp first have your goal HP / KW ATW, then type of application ie drift , drag, street / track etc etc then driveablity ie Lumpyass bump sticks ( cams ) can give you the shits at idle.
Research your turbos with your consideration of application. Ie Single top mount TZ04 etc etc for a dyno queen ( lag monster ) or you want more responsive twin turbo low mount config etc etc....
Reliability - engine built for 900HP but limited at 600HP
cams match your porting in head / exhaust / manifolds etc etc turbo spool / response.....
brakes to stop the f&*ker....
uprated Diff , drivetrain , gear box
stroker kit or 26 built with high rev application crank 10,000rpm tomie / jun fully balanced etc etc can't remeber
anyways I'm sure im just stating what you already know.
rob