John and Mike,
Mike is correct in that the 67th EMM Regiment was a Montgomery County outfit. They came to the Ham Brown barn from Wellsville in the northwest corner of Montgomery County.
Beside the "O.R." reference (volume 41--not 11, but in the 4th part, page 479, as you cited) other sources for this also include:
--the 1937 history of Audrain County, p. 84;
--the 1994 edition of the Daughters of Union Veterans "Missouri: Our Civil War Heritage," pages 9 & 10;
--period newspaper article "Fight in North Missouri," Columbia, "Missouri Statesman," 11 Nov 1864.
Of these, the newspaper gives only a brief summary of this action, but omits the cold-blooded murder aspect of this killing. The Audrain County history account is closest to the 1914 "Fulton Gazette" account that you quote, but I recall that it does not have all the detail of the 1914 version. The Sons of Union Veterans gives a southern participant's verson which includes the murderous nature of the killing of the seven. I found this to be refreshingly accurate for a "victor's" version of this incident.
Incidentally, the seven dead were not all from Callaway County. I think some were from Boone and Audrain Counties as well, based on my search of the 1860 Missouri census.