Heck, find a nail on the wall in the basement and lay that steel straight edge across the top (hinge line) and pull down gently on either side with the nail in the "V". You'll probably have to deflect it slightly past flush to get it to end up flush. If you're worrying about collapsing the "U" channel (I would) wrap a finishing nail in the channel first with wire for as far as you can across the gap. It will keep it from buckling. Go slow or you'll pop the channel out of the epoxy.

Heck, that big a gap is within my acceptable building tolerances. The hinges will pull it up flush when you glue them in if you press the edges together.