WRT "there is no issue at all with having big heads on a small motor"

Ford made that mistake with the Boss 302, which won't pull the skin off a chocolate pudding. The cure: 351" by stroke increase.
Harley-Davidson did the same thing with the K model in 1952: a 750cc twin with 1.56" intake ports, 50% larger than needed. The cure: 900cc by stroke increase.

Missing from the "cfm" rule: "as long as the minimum air speed in the required RPM range is about 200 f/s". Adjust the minimum depending on the application (LSR can be slower than oval).
A 3" sewer pipe flows a lot of cfm, but useless on any automotive engine.