A) Split with Split Polygon tool and snap the resulting vertices horisontaly. Use Append to Polygon tool to fill the hole. Or select the top edges and do Fill Hole.
Download Poly Tools from AW and use Cut Plane. Fill the hole.
C) Get Bonus Game from AW and use Chamfer Vertex.
D) Find out any of the other fifty-fore possible ways to skin this particular cat. 
/Sibben