I know the V8 damper produces better results than the original (or none), but the actual tuned RPM doesn't align with the bad patches in an L6 except by coincidence.
How fast the motor turns only determines how many vibration points it will reach, not where they're located.
The factors that determine this are mostly crank length (number of cylinders in a row, number of main bearings) and stiffness (journal diameter, journal overlap, stroke length), and a SBC is much shorter with the same bore pitch, 2/3 the # of cylinders (+ 1 rod width, etc.) in a bank than a 250. This extra stiffness moves the RPM points wayyy up on the scale, which means the 250 will not reach most of them (and may not line up with any).
To actually predict how many do, I'd need the resonant frequency of a 250 crank in Hz (the 194, 215, 230 cranks will be almost identical, the 292 slightly lower).
Just a guess: if the 250 is 230 Hz, its critical points will be the 3rd at 4,600, 2-1/2nd at 5,520, and 2nd at 6,900 RPM (the others are weaker). Even a long, weak V8 crank (Buick nailhead) is over 300 Hz (up to 450); let's use 350 Hz which gives critical points with the 6th at 3,500, 3rd at 7,000 and and 2-1/2nd at 8,400 RPM (the 2nd is too high).
There is a nice match at 6,900 - 7,000 but it's the only one, and it depends on the data. The entire range from 3,000 to perhaps 6,700 has no matches.
