Lauren my dear, dont worry about changing the Z31 to rb25 - just find out exactly what it has right now and get it optimised - be good to know exactly what that thing has......
plus you already have the 33 with a nice 25 about to go back in! :slap:
However, if you wish to do it, id say all you would need is the Z31 200zr crossmember, talk to RB20ZED on the antilag forums im sure he can point you in the direction of one.
Oh, and a RB25!