I think I remember the cable centerline being about 37mm from the centerline of the steerer. It would be ideal if you could tap threads M6X1 in the hole or "spot face" the location on the hole so there is something "square" for the cable stop to rest against.

I think the clearance hole for M6 is 15/64" as 1/4" is too big.

I am assuming you are talking about a cast aluminum stem. If you are talking about a tubular steels stem, you MUST weld/braze a tube through the hole you drill. DO NOT be tempted to just drill a hole through the middle of a steel stem just drop in some type of cable stop. This could cause the stem to shear. You don't want this.