to fit a rb25 in a s13/14 you simply have to swap the lower subframe and front crossmember from the donor car to the 200sx, things like the steering rack and stuff can all be swapped over with minimal hassle, only probs being getting a new propshaft made up, sorting the rear diff so you get a decent ratio set still, wiring and mapping of course, rb26 is still similar just a little bit more fiddly due to the extra turbo
the thing what puts most people off whois not put off by the amount of work (which granted isnt very much but a big step for most back yard mechanics) is the cost of the transplant, the engines are still expensive and you want the whole lot, practically the whole front end.