Need to scale 340v signal to 3v

I started out writing a question about generating 350v to power a high voltage opamp, but decided it makes more sense to ask for help with the real problem, rather than deciding how to fix it and asking for help with the details. After all you guys are the analog experts and I certainly am not.

I need to monitor a signal that mostly stays at 340v but occasionally drops down to 100v or sometimes to zero. It has some signal content but I think a bandwidth of 2KHz probably covers it. Without loading down the signal, I need to scale this voltage and feed it into a 10-bit ADC on a microprocessor that has a 3v input range. My thought was a 350v opamp with a gain of 0.008, followed by an isolation amp for safety. But it seems like this approach means a 350v power supply for the opamp.

Any suggestions appreciated.