Good idea to look at the actual gravestone inscriptions, and the placing of nearby stones (as to whether they are in the same plot). On worn inscriptions a "6" can look like a "5", for example, and an "8" like a "3". If you take a torch with you, and shine it sideways across the stone, it can make it clearer.
And there is always the possibility of human error with any transcripts!
How old was Mary in the 1850 census (and 1860?), and have you found the family on a ship's passenger list to the US?