The 2008 K-9 tank fits the 2006 K-9 / 2006 RIDGEBACK /2008 RIDGEBACK as well. That has a 4.0 gallon tank
The 2008 Pitbull has a 4.6 gallon tank, but shows that it fits that bike only.
Now ... that being said, it all depends on the level of work you want to put into the modification. Just don't forget the tank height and length (I don't have those specs) when deciding on the change.