Well, the good news for you is that all RB series engines fit right into the S13's (240SX to you) - I'm not too sure about the conversion process myself as i haven't really looked into it that much, however if you PM Nismo_Boy he may be able to point you in the right direction as him and his father are currently putting a RB25DET into a S13 Silvia (the 240SX's sister car - same car different front and back panels).
Also perhaps go to www.japanesemotorsport.com.au and E-mail them as they have done quite a few RB conversions from memory. It's not TOO difficult, however a RB26DETT engine will set you back a heap of money. Also, the RB series engines are a heavier engine, and will effect the handling a fair bit. If you're in the USA and have a 240SX with the KA24DE i suggest you look into a SR20DET conversion as its more cost effective and the easiest conversion you could do. They're also a great engine and it wont upset the cars balance, where as the heavier RB series engines will.
Achieving 400hp out of a SR20DET isn't that difficult either, they're a fairly easy engine to tune. Infact, it'd probrably be alot cheaper and easier than if you were to do it to a RB25DET for example.
hope that helps you a little
Ryan