you can engine swap a v8/RB/1 or 2js/L28et, but just remember IF you have a nice 240z and have any intentions of seling it, ANY swap will likely cut the value drastically. OE cars are fetching premium amounts, modifications are very undesirable
don't forget that its not just an engine swap assuming you want more power, but brakes, suspension, tires, body stiffness all need to be upgraded if you plan to dump in a lot of HP.
So just wondering you don't like the way it drives now with the engine you want to swap out?