I'm guessing that feeding it 450V DC gives a reading around 320V. This is due to it reading the peak AC voltage. The only way to get it to work properly is to remove all the components and replace them with one series resister. The resistor value should be the existing resistor value times 0.707. This is again a guess but if it does read 320 instead of 450 then probably correct. More info on the components, how they are connected and the meter would be useful.
Edit, you want a resistor at least as physically big as the existing one. Preferably bigger (Wattage wise).
Mike.