just bought a ninja backbox from scoobyworld arrived today went to fit it this afternoon about a foot and a half to short does anyone know if u have to change the centre pipe aswell for the backbox to fit,ps my car is an import and the exhaust that is on is double the size.

 

Sounds like you have an aftermarket Jap system on. They often use none standard length centre and backbox pipework.
Normally a Ninja will fit a standard or UK acquired decat centre pipe, you'll need one of these for it to fit.