It's interesting to note that the bigger valves only had a limited run of about 3-4 years. Maybe the engineers figured out that the convoluted gas flow of the stovebolt head just couldn't benefit from a slightly larger valve size, or possibly the parts department logistics of supporting two different valve sizes was just not practical? Other modifications such as reshaping ports and increasing the efficiency of the exhaust side of the equation will probably be needed to fully realize the supposed improvement. It's also possible that the only measurable power gain happened at RPM ranges the average street driven engine rarely, if ever ran.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!