I was planning to do this in my gtst but have since bought a gtr so it can hold down the power. I have 90% of the parts needed to do the conversion at home to suit a r33 gtst.
Think about big picture..
You will need;
Custom cooler piping,
ECU,
Custom wiring,
Exhaust (if needed, stocker Gtt wont cut it)
Custom heater hoses,
New accelorate cable,
Rb25 engine brackers & mounts
Modified Rb26 Sump (cut the diff off, and weld all the holes up)
Rb25 gear box or modified Rb26 box
New boost controller (unless your running a single)
Hope this helps.
J