You did an excellent job. However, the front portion, up to the horizon of the trees in the front of the mountains, are more saturated and contrasty than the mountains in the rear. This can be seen by measuring the saturation with the info pallette, where there are significant differences. It can also be seen in different channels, especially the blue channel. This makes me suspicious that it wasn't the sky that was replaced, but that the picture is actually a composite of two pictures divided at the center, or horizon line.
This may be explained away if you had manipulated the pictures' foreground. Otherwise I'd say this is a good fake, and a little Trutu trickery. Nothing personal, I could be wrong, I've been wrong before.
Good job on this,