The second sideband is resonant in the arms for a cavity length of 37.9299m.

The nearest antiresonant arm lengths for f2 (55MHz) are 36.5753m and 39.2845m.

If we don't touch the ITMs, and we use the room we still have now on the end tables, we can get to 37.5m.

This is how the power spectrum at REFL would look like for perfect antiresonance:

And this is how it looks like for 37.5m:

Or, god forbid, we change the modulation frequencies...