Your dimensions are fixed, well OK but you can stud out and install the dry wall with a 13 degree change from the front of the room to the back of the room. Do the trig, set up a chalk line and install your base plate and studs on the chalk line. With a sloped ceiling and walls with a taper front to back you will end up with almost no parallel surfaces, (front to back wall) and the front and back can be easily broken up with treatments.
jeff
jeff