Hey Luke
Best replacement for an RB20 gearbox would be an RB25 as bubba said.
As far as getting 300rwhp goes, its fairly achievable but just depends how much you want to spend.
I'm going to be aiming for the same sort of power and this is a rough idea what you would need.
- Bigger turbo, and there is a forum member, SLIDE, that does awesome highflows for a great price. That will be enough to get you there. http://www.skylinesaustralia.com/forums/in...howtopic=153972
- 3" turbo back exhaust, could easily find on 2nd hand on the forums
- Bigger fuel pump, also can be found on the forums. Cheap replacement like a WALBRO, or something decent like a 2nd hand GTR pump, or a BOSCH 040 is pretty cheap new these days (roughly $240), will be sufficient.
- Front mount intercooler. There are some alright kits going on the forums. Justjap do one for $345 + postage. 600x300, that includes all the piping. Pretty good upgrade.
- Some type of engine management, so either getting the standard ECU retuned, or getting a Power FC for example, or a WOLF, HALTECH etc etc that will do the job! Plug and play is probably the best option, so something like WOLF 3d or a PFC, as you wont have to pay for installtion, and you can use all your existing sensors without chopping up your loom.
Stock injectors will take you there, and an aftermarket clutch wouldnt hurt. Something like an EXEDY HD brass buttom can take up to 300rwkw, will set you back about $400- $500 new off the forums. Not installed obviously.
RB20 box will be strong enough, depends how you treat it, same with stock clutch.
Have a read of the thread below, it shows some power figures for RB20's and the modifications done to achieve those figures. Will give you a good idea of what works, and what doesnt.
http://www.skylinesaustralia.com/forums/in...howtopic=103095
Just a bit of information that is useful, IMO. Good luck with it all, keep us posted.
Abu