wrote:
this leads me to a very simple question: on a standard day, without
any winds, what would be the optimal altitude for [cruise] speed in a
normally aspirated airplane? is this best altitude dependent on
aircraft to a first-order, or is it fairly constant across airplanes?

I pick the "full-throttle" altitude, usually between 6000 and 7000 feet
MSL, depending on temperature and the engine.