1. What awesome nismo gauges? are you talking 300km/h speedo or something?
2. GTR gearbox has a transfer case for the 4wd. An upgrade for you RB20 box is an RB25 box out of an R33.
3. I doubt a 26 head will fit and why would you want to? 300rwhp out of an RB20 would need more fuel, bigger turbo, bigger injectors, management etc. Would be easier to start with an RB25..
Just to give you a bit of an example.. my car has a front mount cooler, split dump pipe, 3" exhaust and an RB25 turbo running 12.5psi. As far as engine work, this is all I'm doing to the RB20. I am saving up for a 25 engine/box/halfcut as the money on new turbo etc would be better spent with better gains on the RB25.