If you build a soffit to cover ducting, piping, or other obstructions, it is important to note that the lighting installed in the soffit should be placed far enough away so that it does not interfere with the finished racking and/or ducting depth, including depth of crown molding.  You will want to ask for the final depth of your racking including the crown molding at this location and then make sure to allow for the size of the ring on the light fixture as well.  A rule of thumb is to leave a 1” gap from the front of the crown molding to the closest edge of the ring on the can light.  You also want to make sure to use IC can lights so you can insulate around them.