P.S. Even if you wanted to keep exactly 60 cards, I personally would get rid of one of those Sliver Legions and add a Sliver Queen.
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You do realize that a deck doesn't need to have exactly 60 cards in it, right? The MTG official rules have this to say: "100.2a In constructed play (a way of playing in which each player creates his or her own deck ahead of time), each deck must contain at least sixty cards.... 100.5. There is no maximum deck size."