I don't think it too safe to use a 400w ballast for anything other than a 400w bulb.... i say this because you said you have a 250w bulb and then you ask about using the 400w ballast. Would you really want almost double the wattage going through something made for half that much power?
but if you talking about using the ballast w/ a different reflector it'll work just fine, just use the right bulb!
i don't believe there is any "safe" way to put 400watts through a 250watt bulb.....you would have to retard your ballast some how and make it only put out 250w of power to the bulb.....