Find another shop, even if you have to pay them to rework the head. If these guys can't get it right after a couple of tries, it doesn't matter how much the job does (or doesn't) cost. Putting icing on a turd doesn't make it a cupcake! It sounds like their quality, or work ethic just doesn't pass muster. If that valve seat is damaged, or the guide is worn egg-shaped, it's going to take some machine work that goes beyond the average valve job.
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!