The voltage referrs only to the fixture's input voltage, not the voltage seen by the lamp. IIRC merc vapors operate at around 80 volts once the arc is struck.
And the 250 watt bulb in the 400 watt fixture is an extremely bad idea. If you're lucky all that will happen is the lamp won't light. Otherwise you could easily have bulbs exploding or ballasts going down in flames.
Either get the right bulbs or get rebuild kits that will have the bulb, ballast and any other essentials.
