The LM 5116 is rated for 100 VDC input. Better yet, go the National Semiconductor website (power management) and enter the parameters you want to design a simple switcher.
I looked at the datasheet at http://cache.national.com/ds/LM/LM5116.pdf and it had a diagram of a "typical application." I'm wondering what the specs for the resistors, capacitors, etc. are. Any ideas?