ADC analog pin connection
The ADC parts you are using are recommended for 3V operation not 5V (which I am presuming what you are using, given the 5.1 volt protection zener and attenuation ratio). LT say "For 5V specified devices, see LTC1286 and LTC1298". 

Using a zener at 3V is going to be a bit sloppy (due to zener ineffectiveness) so I'd recommend a schottky diode clamp OR choose R2 to be a bit bigger so it can safely attenuate to a voltage within the input range of the ADC.

A low pass filter is probably a good idea - how quickly is the 400V input changing? How fast are you sampling? What are your expectations of performance?