Drill and tap the offending hole for a 3/8" bolt and plug the hole with a short 3/8" bolt installed with LocTite. Cut it off flush with the top of the head. Then center-drill the cutoff bolt and tap it for the correct 1/4"X20 valve cover bolt. Just don't drill all the way through the 3/8" plug with the tap drill. Problem solved!
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!