But then its not a Skyline, and I want my Skyline to be a Skyline. That is all your aircon and alternator and dash wires etc etc etc all plug in and line up. I've had engine conversion cars before and while the 'other' motors performance was better, it was a PITA when you went to o anything like turn your headlights on or believe the speedo and dash or use the aircon and even stuff like lining the gearbox and rear diff up so that you didn't chew out bearings and seals - its never that simple. I'm way over ever doing that again.
However using the 3.4 you don't have any of those issues because its the same motor by the parent company but uprated. Everything fits, the engine mounts, the clutch, gearbox, wiring, you name it, it fits. And the cost may be a bit more [but not that much more than a normal RB25/30 with forgies etc] to start with, but you will get that back and then some in reliability and not having to mod or 'bend' other parts to suit.
Of cause others will disagree, but it works for me.