Oi mate. Just FYI, the capules on an SM57 and an SM58 are nearly identical; there is a slight different in the very low-end response (SM57 goes down to 40Hz, whereas a 58 only goes down to 50Hz) and also a slight difference around the 1.5K range but it's so small as to be negligible.
The drummer in one of my bands here in Chicago is an electrical engineer for Shure, so I know what I'm about to say is extremely accurate: the main difference between the two mics is the ball screen on the 58. Shure coats it with a special coating that changes the way the source sound reaches the capsule; the source sound bouncing around inside the ball screen is what makes the biggest difference in preceived sound between the 57 & 58. So, if you just unscrew the ball screen from the 58, then BAM! you basically have a 57, with only the differences I mentioned above, which for snare mic'ing, are entirely negligible. But I'd recommend using the uncovered 58 on the underside of the snare, as the capsule will be exposed and will not take kindly to being whacked with a drum stick.
Hope that helps.