The stones are listed by the order of a walk through, as you mentioned, as best as Alex was able to do, with some stones not being in a row.
There is a plot map at the back of Vol. 1 of Gravestones of Glengarry for the Williamstown Cemetery. The location of the stone is listed with the inscription in the book and can be located on the plot map. There might be more stones placed between the ones that were copied in the book as it was done some years ago but there are also stones listed that are not there to-day.
Alex has copies of this book available if anyone is interested. See www.glengarrycounty.com/gofg.htlm