450 cradle has more room for the bigger engine.
There are more detials, and 09 250 is different than 06-08 250.
I have AF with 07 kx250f frame. There's a thread about it here.
If you are planning to run a larger tank in the futere.
Use it with your build. I have to make new radiators.
Almost all self build af's are custom.
Dutchie