a mate of mine is putting a RB30 in his AE71 atm using an RB20 box. he's had to cut out the trans tunnel right up to the cross brace in the middle as thats where the shifter sits. the rb20 and rb30 blocks are the same size.
even to get the motor to fit exactaly 10mm from the firewall and 20mm from the rad support the engine mounts had to lean the engine 70mm fwd then stock mounts.
it's really not worth it unless you do all your own fabrication and don't plan on it being legal.
think about the lack of powersteering too.