Can this 1000W ballast be wired for 240V and plugged to 120V to run a 400W MH Lamp?
If you are sattisfied with the result then it's OK.  The higher wattage ballast means just that it has more thicker copper winding and more steel, while the inductance is the same, I think. But you also have an autotransformer that brings you the possibility to use on different suppy voltages.

To be short: I have a ballast of 1kW and I use to connect 250W lamp, fixed voltage, no autotransormer. There is no need to use lower voltage as rated unless you want to have dimmer light.

I'll admit, I was wrong. Of course the inductivity is different, so you'll get higher voltage on the smaller lamp.