Is the tenon on the mil spec standard or wide?
I was looking at Millett's site and they mentioned 2 different widths.

It's neither.  Springfield's have a proprietary tenon width that is about halfway between Colt's wide and Colt's narrow:

-Colt wide:                    .125"
-Colt narrow:                .058"
-Springfield:                 .082"

A wide tenon will work if you narrow it.  You can use a file, but you must be fairly precise, or after staking, it will eventually work itself loose.  When doing Springfield's I usually narrow the tenon on a mill.  King's makes Springfield, or medium tenon sights.