Ed, this is a job for the "Cutting Plane"! Da da da daaahh!. Sorry. No coffee yet this morning.
Actually a cutting plane is just the ticket for this. You draw a plane that cuts through the molding at the desired angle. Select it and the molding. Right click and choose Intersect and then choose Intersect selected (I'm assuming you have GSU 6 other wise just choose Intersect with model.) Delete the waste and the part of the cutting plane you no longer need.
Use Guides (aka Construction Lines) place with the aid of the Protractor tool and Tape Measure tool to give you the correct angle and placement.