Which is: the distance between the inner edges of the upper rails is more critical than the horizontal width of the rail.
Off the top of my head Cephus's numbers are right; I seem to remember shooting for about .2 in. for the horizontal width of the rail.
I've taken the center pin and brought it in just a touch, keep in mind that you're dealing with a fulcrum located at the bottom of the receiver so you don't reduce the length of the center pin the total amount for which you are compensating.
It all worked out and I couldn't tell you now which one it was I did it on.