The best RPM for any engine will be dictated by a number of factors (cam, heads, intake, headers, main bearing size, rod length, valve springs and others).
I was riding with a friend, in his AC Cobra, and we got caught ona two lane, behind an 18 wheeler doing 50MPH. He decided to pass the truck and by the time he got passed the nose of the truck, the speed-o was reading 135MPH. I was grinning from ear to ear.
His engine dynos at 705HP and 695ft/lbs of torque at 5500RPM. Running 3:73 gears through a T56. Sixth put his 80MPH cruise RPM at around 1,900RPM, which was fine for his engine. Oh, it is a 582 cubic inch Robert Pond engine. It is in a pretty mild state of tune.....relatively speaking.
Cubic inches,...oh yeah, definately the way to go.