From what I understand the RB25 head will go on it but needs a bit of work to make the water/oil passages to line up, an RB26 head pretty much slaps right on but you need to either make sleeves and use RB25 head studs or tap the block for the larger 26 head studs.
You would also want to change the rods/pistons because the compression ratio would be way low with just the RB30 bottom end stock (I believe).
You also need to get a special timing belt and setup the tensioners differently than how they are stock, some people use a modified tensioner/idler pully setup and others use a twin tensioner setup.
Here is a good place to start looking for info, also Skylines AU has quite a bit of info. http://meggala.com/RB30head.htm , http://users.on.net/~sandr/twincam.htm and http://paulr33.skylinesaustralia.com/docs/RB30_DOHC.pdf
The hardest thing seems to be finding a piston that will yield a decent compression ratio. Places like SPOOL will sell you Arias or CP Pistons but they might be a custom spec.. RIPS can also provide the pistons you need, as well as the belt, tensioner setup and everything else ya may want if ya wanted to build it stateside.