I don't want to be critical of your plan, but what you're suggesting could seriously result in your house catching on fire.
If you're going to put a 400W light in your attic AT LEAST build some sort of box for it. My 400W HPS operates at almost 400 degrees fahrenheit. I know because I measured it with an IR thermometer. The transformer is also QUITE hot. That's right around the flashpoint point of a lot of stuff in your attic, including wood. In other words, if your light runs just a little hotter than mine, wood could catch on fire just from touching that bulb. Plus, it's in your attic. Even if you happen to be home when the thing catches on fire, it probably will have been burning for a while before you even realize it.
You also can not seal it up in a box. If it's going to be in a small plywood box, you will also need a lot of air going in and out of the box. If you don't have good ventilation, the bulb will turn your box into an oven. Think easy-bake on steroids.