SM57 and SM58

Last I heard, unless it was changed very recently, the SM57 and SM58 are the exact same mic with a different head casing. I'm not sure what kind of an impact the head casing on the 57 has, but if you took that ball top off the 58 they would be the same thing i guess except for that screen on the end of the 57.

I've never tried comparing recordings between a headless 58 and a 57, so I can't say how close the sound you'd get would be.

Here is what the Shure Knowledge Base says on the subject:

It is true the SM57 and SM58® microphones are based on the same cartridge design. The main difference between them is in the grille design. The SM58 was designed for vocal application and it uses a separate grille with a very effective pop filter. The SM57 was designed as an instrument microphone where smaller grille size is preferred. In this application the pop and wind are not usually a concern. The SM57 uses an integral resonator/grille assembly, where the grille is actually a part of the cartridge. These two grille designs place the diaphragm of the microphones in a different acoustical environment. First of all, the distance from the top of the grille to the diaphragm is significantly shorter on the SM57 compared to that of the SM58. This allows for closer sound pickup with even more pronounced proximity effect. Second, a different resonator/grille assembly design of the SM57 is responsible for its slightly higher output above 5 kHz.

They will not sound the same, no matter what you do. The 57 has a resonator as part of its capsule, the 58 does not. Not to mention the fact that dropping a 58 without the grill on will be a very bad thing...

Something similar to the SM57 would be the Electro voice EV04 ?( I think )
that looks like a 57 a bit shorter and stouter than the 57
for slightly less cash ( if you can still find them around )