These 304 Stainless Steel V-Band Clamps are made specifically for exhaust systems. Other brands will sometimes use a nylon lock nut, which instantly melts when used in an exhaust application.
This is just the clamp, sold for replacing damaged or missing clamps from your exhaust system.
For the Spec E46 exhaust, use the 3″ clamp. Most E46 M3 race exhausts (such as the one from BimmerWorld) use the 3.5″ clamp.