well now that you've put a little more thought into just "can you drop a v6 into a 240?", here's my more serious answer
If you ever taken a look at the 300z engine bay, you will notice that there's VERY little room to get your hands here and there, especially with the twin turbo model. So if you were to swap that engine into the 240sx body, there will definitely be issues regarding how much room it will take up. Furthermore, it'll throw the weight balance of the car forward, and that ruins the whole point of owning a 240. If that engine was an inline 6 (which by the way, is the reason why I don't like the z's because of its v6 engine configuration and little room to work with), then MAYBE its more feasible to drop that engine into the 240.
Even if you manage somehow to get that 300z (vg30dett or whatever it is) into the bay, you will still need to fabricate motor mounts, tranny mounts, bell housings, driveshaft, etc etc. By the time your down doing all the fabrication needed on the 240, it'll perform as a totally different car.
As for which nissan v6 engines, the only feasible one would be the 300z's since they're the only one that's designed for a rwd application.
For those american V8's you see, well the car is already designed for a v8 in mind, so there's plenty of room for ANY kind of engine as long as you can make the mounts for them.
I haven't seen a 300z engine in a 240 yet, but I've seen plenty of rb's in 240s, rb's in older model datsuns, and even sr20's in datsuns.