Something to keep in mind. There is a huge difference between the RB26 and RB25 motors.
the biggest is strenght and the head. the RB26 block is considerablly stronger and the factory head will out flow even a decently ported 25T head.
About the only thing they share is the piston diameters which is identical. As a matter of fact RB26DETT pistons drop right in.
There is quite a bit more in differences but I see no need to go into it deeper.
As far as putting one in a left hand drive 240sx if the exhaust manifold is an issue I am sure you can have one made up to keep it twin turbo. I don't see how the steering rack can cause and issue since there is a real SILEIGHTY with that same transplant done using the standard plenum which sticks out even more than the manifold with turbos on it.
There will need to be a fair bit of extra done to the car such as wire looms, ecu, fuel delivery system, and all the pluming for the front mount you will need.
There is also a strength difference in the transmissions. The GTR 4wd trans is stronger than the 25T box and the gear ratios are slightly different. Remember you also would not be putting as much of a load on it because unless your running drag slicks you will be spinning the tyres over quite hard thus lightening the load on the box.