i have change over my car from l series engine and box to rb series.
now the mounts should be the same, gear box spline is the same..... ( was able to use the same tail shaft on mine), need to check to see if you have room at the top on the box near the shifter. the l series box is egg hspped cross section ( narrow at top), while the rb series box is ver large and square( i had to stretch my trans tunnel with a hydraulic porta pack). the only thing that may be an issue is that the bell housing will probley be different. but i have heard there is adaptor bell housings that you can buy and change over ya self. the slave cylinder for the clutch is the same ( get the rb one if ya can, then seem to look stronger)
i hope that helps.........
Pete.