Is the bad valve one of the ones where the seat was changed? I had a really bad experience once with a Plymouth Roadrunner 383 when a replacement valve seat came adrift on a dyno run and wrecked a piston. It turned out that the press fit on the seat was not sufficient to hold it into place, and when it came loose it broke up into pieces in just a few cycles of the valve. We salvaged the engine, but one cylinder head got trashed and a piston with lots of custom work and special balancing had to be replaced. My fault- - - -I mis-read the micrometer by a couple of thousandths when I counterbored the hole for the replacement seat! Those mistakes can get expensive!
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!