You have to not just have +3 a-arms, but have +3 LT's (long travel) a-arms.
If you have ST (standard travel) the lower pivot point is closer in towards the frame and the longer YFZ450 shocks will cause the suspension to bind at the farthest droop point.
The advantage of the YFZ shocks is that they are equivalent to "stage 4" shocks. They have threaded preload, progressive spring, compression and rebound adjustment, and are rebuildable. For stock shocks, they're AWESOME.