The RB25DET is the way to go.. The RB20DET is outdated and very hard to get parts for if it ever breaks. The RB25 is a lot stronger too.. You can get an install kit from McKinney Motorsports. They have a kit to install into an S13 chassis. Comes with Motormounts, driveshaft, tranny mount and custom wiring harness to adapt into your S13. You have to send them your RB harness as a core. I see you have an S12... but you listed a 240 in your poll... so I don't know what you plan on putting this in.