This is most likely because there are no FACTUAL bonuses to putting an RB25 into an S13....

About the only thing you get to say is that

A: you make more torque at lower revs than an SR20 (which is offset by the extra weight so it dosent matter)
B: You can say u have a skyline motor..

Big freakin deal it seems to me... If it really means that much to you to have the bragging rights then thats ok.. I just have a pretty solid feeling that after u race some of the Secret Services SR powered cars your going to be thinking twice about all the $$$ you spent on an RB....