try 240sx.org, its been discussed to death. i was in the same dilema as you a few months back. what do u mean by new driveline for the rb25det? new driveshaft? i believe u can use the rb25 one if it comes with the clip, same for the rb20. It pretty much comes down to if you want to be different, if you want 530 whp with room might as well go rb26dett, forget the rb20 at least.