Swapping a RB motor into a Z is ill-advised. The torque curve of a VG versus a RB motor is substantially in the VG's favor. The driveability will be far less enjoyable in a heavier platform like the Z. That being said, if you had the financial means to do this swap, then you must indeed have a great deal of money laying around. The RB is a great engine in itself be it the 26, 25, or 20. The RB25 would be a far poorer choice then say the RB26DETT for some obvious reasons, mainly under the curve power output. Save yourself the headaches and research the motor in your vehicle. In my humble opinion, learn about your VG and what it truly is capable these days with the assistance of good reputable tuners and high quality upgraded modifications.
1992 Nissan Skyline GT-R Gunmetal Grey - Long Live the Z and Godzirra! - "Love cars. Love people. Love life." - Mr. K.