I gave a person smarter than me data from the factory service manual as to measurements, weight, CW diameter etc.
He has an expensive program that approximates the frequency, and that figure is about where you would expect to find it vs. a shorter stroke 7 bearing engine (like its replacement 250).
Quoting myself (from my book):
"I suspect that a crankshaft may be tested to detect its natural resonant frequency by simply treating it as a tuning fork: strike it smartly with a brass or lead (soft) hammer. A microphone and oscilloscope will show the frequency of the “ring” sound. Different locations should be almost identical.
Once the frequency is known, the formula* can be used to predict the RPM at the critical orders.
Alternatively, with the crankshaft suspended, a tone generator can be used to excite it by slowly dialing through the likely frequencies (between perhaps 180 and 250 Hz), and observing a fine wire rigidly attached in various places for vibration."
*"L6 engines generally resonate at frequencies between 200 and 300 Hz (source: Heldt). These orders occur throughout, and well above the maximum RPM of the stovebolt crankshaft.
The engine speeds for these orders can be calculated from the resonant frequency, where “Hz” is the crankshaft’s resonant frequency, and N is the order number, using this formula: RPM = Hz × 60 ÷ N
For standard 235 and 261 stovebolts, RPM = 12,960 ÷ N."