Bill,
If you can expose the front of the M60s and have them protruding out from the recessed space, that will be fine. Even better if they don't fire through the acoustically transparent screen. The latter are never totally transparent and do compromise sound quality, but for movies, the compromise may not be all that critical. You should also realize that the perforations in acoustically transparent screens reduce the brightness of the projected image by as much as 20%, according to projection video guru Joe Kane.
As to separation between the speakers, I tend to err on the side of caution. It's not cast in stone, and you can separate them by as much as the same distance as your viewing distance. I tend to suggest "6 to 10 feet", unless you're in a huge room sitting a long way back. It's worth experimenting before you commit to set positions in a given room. As Dr. Floyd Toole points out, "the room is the forgotten component."
Alan
