Back when just about every vehicle ran drum brakes, the rule of thumb for machining passenger car drums was .060" max, and most light and medium-duty trucks had a .090" max oversize. Those 14" drums are virtually impossible to find new, and good used ones are almost as scarce. Don't machine them any more than absolutely necessary. If there's a deep groove, leave it and just clean up the main drum surface area.
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!