Hence my question, because if I leave the throttle firewalled in either
the '67 172 (w/ 6 cyl continental) or the 81 Cherokee 161, the needle
WILL pass redline in level flight.

Perhaps you have a cruise prop on both aircraft, Ben? You shouldn't be
exceeding red line simply by going to full throttle in level flight,
*unless* your prop has been pitched differently than stock.
--
Jay Honeck
Iowa City, IA
Pathfinder N56993
www.AlexisParkInn.com
"Your Aviation Destination"