Because the 426 head won't fit the 450 cylinder.
As to why they put a 450 cylinder on it, I have no idea. I could not possibly guess the motivation for some of the bizarre concoctions some quad riders cob together.
So whats the difference between the 426 and 450 cylinder?
and the 426 head and 450 head?
I think there should be a cross compability chart of some sort for these motors. I think that would help alot of people out.