Over designed heat sink or a well designed one?
You have to change the design of your heat sink or increase the capacity of the fan. Using the dimensions provided and the dimensions of a IRFP460, Tj = 163C in a 45C ambient based on calculations using the heat sink calculator (http://www.heatsinkcalculator.com). The Rcs (resistance case to heat sink) was estimated using a film of thermal grease of 0.1mm thick with a thermal conductivity of 1.35W/(m.K) with the contact area equal to the width X length of the IRFP460. This gives a Rcs value of 0.24 C/W.  

Keep in mind that the maximum flow rating of a fan is based on no obstructions being in front of the fan. Once the fan is place behind the heat sink the flow provided will be less than the maximum flow. The exact value is dependent on the design of the heat sink and the fan flow curve.