I don't think there is anything perculiar to the RB30 engine. the engine builder should have a pretty good idea of what to aim for with the bearing clearances for a performance engine. the piston to bore clearance will most likely be stipulated by the piston manufacturer. remember that any "RB30 specs" you do get will have been written with the intention of the motor being used in either a forklift, a low power low rpm NA, or a low boost fairly low power turbo set-up so they may not be suitable for what you want from the engine/car anyway.
and personally I think an NA RB30/26 will be kick ass. but if it were me I would have scrapped the RB30 part and just build a nuts high comp RB26, or maybe even RB28. now you must post some sound files of it when it's done. I want to hear this thing at 8500rpm!