I have dipole wall mount speakers that were placed on our vaulted ceiling for the rears. They are 6ft overhead and about 2ft behind the couch so they are somewhat pointed toward the back of the viewers head. For my odd shaped room the rears sound pretty darn good!

If I were you, I would use a set of bookshelf speakers to determine placement by trial and error before cutting any holes. You may find that placing wall speakers high in the wall (oriented horizontally) gives a cleaner sound than any placement in a sloped ceiling. Wall speakers would probably sound even better if aimed parallel to the ceiling ridge (not aimed toward the slope, but with it).
Casey

Here is a diagram showing the room. If I placed them in the ceiling, should I place them slightly behind the sofa or in the ceiling over the kitchen to get the proper angle to the TV Viewer?

Here is a link that might be useful: Great Room Diagram