You need to get a 400 watt bulb.
If you want to settle a question for us, you can try the 70 watt bulb and it should burn out or explode rather quickly. You are supposed to be able to use a larger bulb in a smaller ballast but not a larger ballast with a smaller bulb because it is supposed to push the full 400 watts thru your 70 watt bulb. This is what the book work says but no one is brave enough for real world experience.