one pound of rotating mass is worth roughly 6 pounds of static weight

Way too general - doesn't say radius (major factor), or relative speed (engine speed or less?).

the effective weight of the rotating mass is the sum of the crank plus the rotational weight of the pistons and rods. you have 2 cranks one weighs 50 and the other 60 it is 20% lighter, but if you included ONLY the rotating (big end) weight of 8 rods (lets say the same 10 pounds) the effective rotational difference is really 60 vs 70 or a net of 16.6% lighter

The actual math to calculate changes in inertia is a mess - the formula is easy, but figuring out the "mean center of gyration" isn't.
If the crank were lightened 20 lbs. by simply drilling the main journals out to paper thin (this never happens!!), the inertia would hardly change - because their mean radius less than 1-1/4". The rod big end is somewhat better since it's located (about) stroke radius distance. The really important part is the counterweight OD.

I was told that the biggest noticable difference would be in the 60 foot times
True - because the overall gear ratio is directly proportionate to inertia.