I've always just put a ball bearing which is larger than the hole in the hole and tapped it with a hammer and it removes the dish in the bolt holes with a few judicious taps. Support the top side of the cover with a block of wood. You won't damage the outer flange of the cover unless you get carried away with your hammer taps. Unless the chrome is ridiculously thick it should flex along with the underlying steel without any issue.
I've addressed the bolt holes on my original chrome L79 valve covers and the rechromed GTO valve covers numerous times without any problems. An alternate method if you don't have any ball bearings lying around is the put the ball end of a ball peen hammer on the hole and tap the head of the hammer with another hammer.
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