A slot between the valve guide and the head might result in a vacuum leak or an exhaust leak, depending on which guide it is. If the head gets grooved, ream the hole out a few thousandths to smooth it up and use an oversized guide fitted to the hole for the correct press fit. Apparently, our high tech guys have never heard of an adjustable reamer, or anything else that requires a little common sense. They can brag about how much their equipment costs, however.
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!