Ok if you have an rb25 and this is what your going to install all you need is are the motor mounts, oil pan and oil pickup from the rb20 z31 out of japan. this will allow you to drop the motor in and start on mounting the tranny.
You will need access to the japan nissan parts market if you dont have this i would go the custom route.
If you want the motor to sit in its optimal position ... as low and as far back as possible...custom mounts are what you need. everything else above will still work. the oil pan will sit about 1 onch below the A Frame, and will be the first thing to hit if you bottom out.