The gearbox in the Gtr is very very heavy mate!
You need to undo the rear prop and the front one, from the transfer box.
In order to get to the top two bolts you need to tilt the back of the box down and use some very long extension bars. You'll also need to remove the slave from the side of the box too. Plus the two electrics connectors on top, plus unscrew the speedo speed sensor. Oh and the oil line on the back for the attessa.
I don't know how mechanically minded you are but its almost better to pay someone else to do it if you don't have a lift and a transmission jack.
You also need to swing/twist the box as it comes off the engine so that it will clear the transmission tunnel before you can remove it.
It's a real bugger of a job on your own. With two strong people not as bad.
Hope that helps? If you need any more info drop me a pm.