I do not think that a 250W MH ballast will fire a 400W bulb. From what I have researched you need to match the bulb with the ballast. It makes no difference if it is a magnetic or electronic ballast.
Have you thought about running T-5 bulbs on your 40?
I agree with Chris that retro fit is the way to go. You have much more flexibility with what you can do, have a better selection of ballast, bulbs, reflectors etc. You will end up spending the same $$$, but will end up with top of the line components.