Did the bed stiffen up the frame?
Just my oppinion here so dont taske it to heart. Tieing the cage through the hole would be the simplest, but I think its gunna look a bit funny being that narrow. Not to mention it will minimize your bed space. going to the rear angled gussets would look better and it still ties to the frame through them. It would keep more bed space as well. Cousin willards cage is wider than the frame as well.
Other than that its looking great!