Can you tell me if the tank and stock seat is a perfect fit?
It is, I'm quite happy with the swap. I ended up with a different tank, but it is an IMS natural oversized tank.
You can see closeup photos of both in my garage. I have to push on the seat a little to get the rear screws in, but no worse than an aftermarket seat on my friends 250X.
Unabiker radiator guards won't fit with an oversized YZ tank though. I have a thread in this forum on that. Devol seems to fit.