Anh, your far better off just using a R33/R34 GTS RB25det manual front cut.
It'll cost FAR less & be nearly as quick.
If you really wish to go down the GTR path with the 6sp gearbox, you'll need to strip the back off the GTR gearbox, remove the Altessa centre clutch pack, cut the casting off the housing where the centre clutch back was & weld the hole up. This is NOT something someone without experience should attempt.
You may also find major problems with the ECU, when it can't find the sensors for the 4WD system.
Seems like a lot of work for when you add up the 10k+ purchase price for a BNR34 front cut...