Since you haven't had a response in 24 hours, I'll try. I'm not sure what this HP712A power supply is or how it will help get to 600 volts. My Google search turned up info suggesting it was a computer. The power supply for that won't help. If the HP712A is an HP high voltage unregulated power supply, your variac should make it variable. If not, you'll need to start with a transformer that has about 480 volts AC on it's secondary assuming you're looking for an approximate 600 volt DC output. You'll then need four diodes in a full wave bridge to change AC to DC and a filter cap with a bleeder resistor. This is as simple as it gets and may have some ripple depending on the load. It can be improved by adding a resistor or choke between the rectifier and capacitor.
The transformer and diodes must be rated to handle the current the device you are intending to run will require. Additionally the diodes should have a voltage rating at least 1KV. If in doubt, use two diodes in series to form each leg of the bridge. Radio Shack has 1 KV 2.5 amp diodes. Filter capacitors for this job are often a pair of 450 volt 100 to 220 uf capacitors in series, with each cap having it's own 470 K ohm 2 watt bleeder resistor. Fuse the primary. This and your variac should do what you want. Keep one hand in your pocket when working with it live.
One common source for your transformer could be a simple isolation transformer with a 220 volt secondary. Since you will probably use a pair of filter caps anyhow, it's easy to make a voltage doubler with only two rectifier diodes rather then four. Much of this will depend on the parts that are most available to you. To wire either the "full wave bridge" or the "voltage doubler", just Google those terms for images on how to wire them.
