while the ballast does produce a little heat your blower should take care of it the only concern would be the carbon filter may reduce the cfm by quite a bit.
if you are using clones wouldnt really bother with lst that is what the screens for so you can position them under the screen and tie where ever you want.
also might read up on light position for best effeciency while it probably wouldnt make to much difference in your case as you are using a 400 in a small area but the link is below.
http://www.drugs-forum.com/growfaq/1163.htm