1. Your will need custom mounts to fit the RB into a 240SX chassis (untable-hybrids said that they will be releasing those custom mounts to the public for sale for the people that want to do the RB swap into a 240)
2.
3. Yes you will use the RB tranny
4. I don't think there are really any advantages to the RB20DET over the RB25DET. Both of these motors have a crazy amount of potential. The RB25DET will probably be able to be modified to those higher HP numbers quicker than the RB20DET just because of the higher displacement of the RB25.