The key is to determine what the crank's resonant frequency is, and that's also somewhat variable based on the flywheel weight, type (and method of attachment), balance weight removed from the CW, and even the fixed part of the damper (the hub).
The math is very messy, the closest guess will be from an engine with about the same:
1. length, or at least similar config like another L6 with similar bore pitch. A 235/261 vs. 270/302 GMC will not be too far away, the 235 will be slightly higher Hz (shorter length and stroke, same journals = stiffer)
2. stroke length
3. journal sizes
4. number of main bearings (not directly, but it affect where the CW are)
I got an engineering guy to run an analysis of a 235 crank for me based on factory drawings, material specs, weight, fasteners etc. and it's pretty low (= low RPM for all relevant orders), lower than the 250 etc.
I suspect that a clean crank can be tested for frequency the same way you would test for cracks: suspend it, and smack it with something soft but metallic like a brass hammer.
A mike attached to an oscilloscope will pick up a strong trace at at least 1 point.
Alternative: a tone generator that produces a strong signal between 200 and 350 has its horn aimed at the crank, and spun up through the range. A vibration probe on the CW will jump when the frequency is correct.
