WHAT IS THE DIFFERENCE BETWEEN A 240sx and a silvia??

Lots. Look in the 240SX forum FAQ. Don't try to turn a 240SX into a Silvia, either. No matter what you do, it will pretty much always be a 240SX.

the major things are the engine and front end the chassis are the same but a 240sx can never be a silvia but u can make a 240 look like a silvia and have the same parts but it can never be titled a silvia oh and a silvia is right hand drive and 240's are left. there are many other differences but those are the extremly obvious differences