That's for all common L6 engines with the usual firing orders (not 2-stroke or flat crank). The 2, 2-1/2 and 3 are very strong and also fall into the RPM range you might need.
The 4200 is probably stiffer than most 4 main engines, some L6 engines have as natural frequencies as high as 300 Hz.
To compare similar crankshafts, factors that weaken them (reduce stiffness) and lower the frequency are:
1. small main journals
2. low number of main journals
3. small rod journals
4. small journal overlap
5. long stroke
6. long overall length
7. long distance between main journals
8. low counterweight mass
If there's really a bad spot at 7,200 it may be a harmonic order, but 240 Hz makes it the 2nd order. The 3rd order will make the frequency, IMHO, too high for the engine design at 360 Hz. The 2-1/2 order is 300 Hz, quite possible.
