I don't think you would be able to use this profile but we have never tried so I don't want to completely shoot it down. The shape is like an S so I don't think you will have much clearance on the back of the moulding. You may want to look at part # 23125 if you like the profile of 13070. In order to use this as a valance, you will need to add some additional flat board to the bottom and then create a box by attaching pieces of each side that return to the wall. Here are some pretty simple instructions I found online to make a valance box using crown moulding.

1. Make the basic rectangle for the valance. Cut one piece of flat board/wood to the desired length and two pieces for the sides. Attach the three pieces with glue and a nail gun.

2. Add size- and style-appropriate molding to the valance. Cut the molding at a 45 degree angle, making sure the corners fit snugly. Attach each piece with a nail gun and adhesive.

3. Apply a coat of primer on the valance, let it dry completely then apply a coat of paint.

4. Attach the valance directly to the old window casing with the nail gun. Or you can use an L-bracket to attach the valance to the wall. Make sure the bracket is secured to a stud.