The adoption of the 14th Amendment is naturalization. You cannot create instant natural born citizen from people who were not US citizens before the Act.
I'm no Constitutional scholar, so I will defer to those who are, but I'm thinking that former slaves and American Indians made citizens by the 14th Amendment were also made eligible for the Office of President. Once you amend the Constitution, its original language must be read in the context of the amendment, its no longer the original document. I believe that this was what the founders intended.