Whenever an engine stops, a few valves will be open due to the position of the camshaft and lifters. Those cylinders are subject to draw moisture due to temperature and humidity changes. I'd suggest backing all the valve adjustments off enough to close all the valves, then use a pry bar on the rocker arms to cycle the valves up and down as you soak the guides with penetrating oil. Put plenty of penetrant into all the spark plug holes and wait- - - -several weeks if possible. A 50/50 mix of ATF and acetone is probably the best, and least expensive penetrant you can use.
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!