I would go 5.1 and call it done.
Don't get overly hung up on getting 7.1 installed if your room really doesn't support it. 2.0, 2.1, 3.1, and 5.1 are all just fine if that what works best for your room. Doing a crappy setup because you want it is a far worse decision than a quality setup that is limited, but more accurate.