I would start with the speakers at least 3 feet or so from the back wall given the length of your room. I found the imaging to be a little too random with a live wall, deadening it seemed to focus the image a little more, but that's just me. In your room, I would put the tweeters on the inside to minimize side reflections. There were instructions on that in the manual (I sent my speakers back to Magnepan) governing this. Another thing to play with is the metal swivel on the rear of the stands, you might tilt them down given the distance to the speakers. One more thing to play with is using the attenuating resistors on the tweeter if it is a little bright.
Good luck!
Good luck!