Get a mill to do it!
I have the machinists blueprints in front of me and I would say do it with a woodruff keyway cutter after putting it in a mill vise and dial indicating it. Because it will do it evenly on both sides and a-lot faster to boot.
That is my opinion though. Filing can be very imprecise let alone doing two sides the same is not too easy at all. But if you want to do it more power to you.
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