One thing you have in your favor - since I'm describing a "buck" type converter, full power is achieved with the FET 100% on = almost no dissipation (in the FET). The IRF4905 is a beast - 55V, 74 Amps. I have yet to blow one. A 45V Transzorb across the device and a 15V zener across the GATE and SOURCE is a good idea, though.