I've observed vibration vs rpms too. My stroker with 4.062 stroke, Crower rods, light JE forged pistons, heavy 168 tooth flywheel vibrates at about 75 mph. Smooth at 70 and smooth at 80. Crank and flywheel were balanced together, altered SBC fluid damper on the front. It's noticible enough to make me drive slower or faster to avoid the sour spot.
Not sure what the rpms are in my case. It's a '72 Nova with Muncie, 3.08 gears, tires similar height as stock.
