Don't know if you concerned yourself about this as you designed your muffler, but the speed of sound is strongly influenced by the temperature. The speed of sound in gasses is proportional to the square root of the absolute temperature. (There is only a tiny change due to pressure, by the way.)

At room temperature (20 C), the speed of sound in dry air is 344 m/s. At 500 C, the speed of sound in air is 553 m/s. The exhaust gas temperature would probably be something like 500 C as it entered the muffler, perhaps hotter.

The resonance length will scale with the square root of temperature.