i recently put a neo in my 33 after finding out the crank key way was thrashed out. the big costs are as follows: engine, wiring, ecu, clutch. all up i spend about 3.5k doing my conversion and its still not running right. before you go throwing anything in your engine bay do the water pump and timing belt, so much easier when its out. the engine goes straigt in using the r33 engine mounts, n/a 33s run the rb20 box which bolts straight up and you just use the 33 rb20 clutch kit (excedy). next you need to get it all wired up. buy a 34 engine harness and keep your 33 one. take them to a decent sparky or get them to come to yours like i did and they can wire it all up in a day. thread loom into engine bay and then connect everything up. hook up the ecu and then your down to the smaller problems. problems i ran into:
need to get a custom accelerator cable made up as the 33 one is too short and i belive the 34 one is too long. mine cost 130 bucks. next problem was for some reason my neo runs the 33 CAS. you need to find out which one you need.
the vaccuum hose that runs between the plennum and the brake booster needs a bit of man handling due to the 33 one not being in the same place as the 34 but with a bit of a shove itll work. finally dont let any of your mates that have no clue about engines help out. left one mate the job of doing the radiator hoses. didnt bother checking them cause i thought he could do a good job. 10 mins on the first drive the bottom hose came off. in conclusion its a shit tonne of work but it is nice rocking up to car meets and when people say 'ahh dw bout it its just a boring old rb25de' you can always pop the hood and show them what you did in your driveway with limited help and knowledge.