Voltage rating of low side resistor in voltage divider circuit
I don't see the need for R2 to be rated at 600V. If you lose connection to GND, the circuit will be open and both resistors will have 0V across them.

As a side note, if you find it dificult to source high voltage resistors, you can always use several lower voltage resistors in series. It's a common practice in multimeter design.