One thing to do to when you are replacing a head in a shot is to lower the opacity of the pasted in shot and transform it (resize and skew) to fit the layout of the face you are covering. This way the eyes, nose and mouth are in the same orientation as the previous face.
Then carefully erase the parts not needed. You may even set the eraser to a lower opacity as well. This way you can "blend" the two together.
I think you did a pretty good job to start. The hairline on her neck looks out of place as well as her chin.
I would try the above, and you should be able to get a pretty good patch! You already have the skintones matched.
Let us see the final results?!
Paul