Whats the best way to regulate a PS down?

Is it a constant current, you're drawing from the PSU?
Is it a low or high current, your drawing?
Is the 39 Volt with or without load?

If it's not a constant current, but sometimes a high one, the problem is the small difference between the unregulated and regulated voltage, since the voltage drop-out(that's the the minimum difference between input and output needed, before the regulator's regulating) for a high-current regulator's typically 4-6 Volt.

In short: more info's needed.