I cut the building and pasted it on another pic to change its background! I need help!

I pasted it on another photo of a nicer sky, but I am finding it difficult to edit the fine details of this picture, as you can see, I have tried to edit the top of the building at right by adding a layer mask (black - transperant) and erased the sky between those rods but it didn't work well! The left part is still unedited, you can see the original sky behind. How can I edit it perfectly? Is there any easier way to do it?