I'm a volunteer on stage crew for a bunch of folk festivals, miking all kinds of instruments under the direction of pros. My professional sound tech friends say that the SM 58 has a relatively narrow pickup angle and is used primarily for close vocal work, so it will isolate out sounds coming from the side. As a result, it's not that good for area miking.

Not being one of the above-mentioned pros, I can't give alternative mike suggestions, but I could try calling the guys at National Events for some advice...