The 180sx is not a silvia, your talking RPS13 (180sx) VS S13 (silvia). The difference is in the front end, the hatch, and the engine.
Now, the 240sx is basically a one-via (front end of 180sx and rear end of a silvia). Much like the Sil-Eighty, which was the exact opposite combination (front of a silvia, rear of a 180).
Some Japaneese car guys/drifters first came up with these 2 cars (onevia and sileighty) because when they crashed their cars into the mountain side, it was cheaper to replace the expensive pop up headlights with the silvia's front end. From this, some guys did the opposite to be different and replaced the front end of their siliva with the 180sx's pop up front. Nissan saw this and apparently thought it was a good idea, and they produced the 240sx and the sileighty (although the sileighty was sold in very small numbers and is now a collectors car, you can tell a real sileighty because it's printed on the rear quarter glass and on the center between the tail lights).
The difference of the 240sx being that the motor was the 2.4L KA24DE. (essentially a truck motor)
Now, these three cars are completely different, except for the base unibody and some interior pieces.