re the side walls - you'd have to see the layout - left 8', right 10'.
The front and back walls are more of an acoustic concern - 13' apart and parallel - even slight toe in make a big difference/helps to disrupt the standing wave.
Listening to a 24bit vinyl rip of My Life In The Bush of Ghosts as I type - and I have to say these old M80v2s are so impressive - the idea that the M5HP could trump them is exciting...