Remember the RB series engines have nearly identical bolt patters to the KA and L series. The RB20** used the same series transmissions too. They also tilt to the left about the same 10-15* as KA and SR.
Quote from pic source:
http://forum.r31skylineclub.com/index.php?topic=64635.15
"With the magic of television, I overlay them: SMT798B is KA SMT534A is RB There's the bolt on the right that the KA has that the RB doesn't, as Juls rightly pointed out, but they look pretty close. Oh well, don't know for sure. I am reminded of E series and GA series, they also differ in this way, the E has a bolt on the right that the GA doesn't, but they interchange, I have physically examined a GA bolted to an E gearbox, so I'm not relying on my eyes for that one. Regarding the L/RB thing, I thought there also was a couple of different bolts on there, but you could either lose one bolt or gain another to change, but I don't trust my memory on that one anymore."
Another useful thing may be the shifter location differences in the KA/SR and RB. The RB20** trans has the shifter moved forward about 15mm.