Ok - if you really want a fast track / daily driver R34 GTR then the 1st thing to do is stop worrying about getting to 600HP and start worrying about what going to make it fast around a track.
A standard 2.6L with forged internals and 2530's, cams, injectors, ecu etc will get you around the 500HP@wheels mark.
However the suspension geometry on the R34 is all whack. The standard control arms etc don't allow enough castor to make a decent track car so that will need to be all redesigned. Obviously some good coil-overs are essential
Moddifications to the Attessa and a good mech diff will help drive out of corners. You can also change the final drive and if going extreme add a carbon fibre propshaft as per the infamous Mines R34. Do all that and chuck on some huge brakes and you'll have a pretty serious piece of kit.
However - if all you really want is 600HP then put on a single.