its a n200 diff buddy get any s13,s14,s15 or r33gtst 2way and it will fit into your casing
i dont think ive seen one in a s15 casing for a long time
i had to build mine
ratio depends on what you want out of the car,
if you want top end speed then you want to change your 3.9 for a 3.7 or so on
or if you dont think your ever going to see 140+mph but want more acceleration/ torque then lower your 3.9 to a 4.1
personally im a fan of most of cusco's parts as the quality is there, my nismo 2way was nice or most people use a kaaz
to be fair you would be better of buying a second hand one as its alot cheaper, and paying sombody to fit it to your casing