Multiple isolated DC-DC converters in series - ground reference question
You already have correct answer by @Aaron in the comments. This is just my two cents on the implementation.
The ADC on the source side is probably most cumbersome solution possible. You would need additional power sources for ADC chips and digital isolators. The one questionable benefit of this approach is somewhat higher measurement precision.
Isolation amp is somewhat better, but it is not easy to find isolated amplifiers for so high supply voltages on both sides, not to mention single supply rail capability.
IMHO your best option is to use simple optoisolator circuits. They are much easier to adapt to variety of the source voltages, and by changing their gain you can have two identical ranges on the outputs suitable for Arduino ADC.
The only drawback of this solution is that for better overall precision you need to ensure you are working in the linear range of the particular optoisolator chip, which may result in some non-linearity at the extreme ends of the scale.