Nancy may have died in Missouri and might be buried there.
For the time being, I have posted her in the same cemetery as her parents and son.
She is listed as Rachel on her son William's death cert and in the Peter Hart book by Ruth Gibbs Hart and Karen L Cooper.
Nancy is last found in Missouri in 1880 and likely died about 1885. John and some of his children must have returned to Kentucky soon after that as he remarried "Belle" Floyd in 1886.

1850 United States Federal Census
Name Nancy A Floyd
Age 7
Birth Year abt 1843
Birthplace Kentucky
Home in 1850 Knox, Kentucky, USA
Gender Female
Family Number 695
Household Members
Name Age
William Floyd 49
Nancy Floyd 44
James Floyd 21
Stephen Floyd 15
Mary E Floyd 13
Susan Floyd 11
Jane Floyd 9
Nancy A Floyd 7
Winny Floyd 3
Mary Bains 22

1860 United States Federal Census
Name Abagale Floyd
Age 15
Birth Year abt 1845
Gender Female
Birth Place Kentucky
Home in 1860 Lynn Camp, Knox, Kentucky
Post Office Barbourville
Family Number 50
Household Members
Name Age
William Floyd 60
Nancy Floyd 54
Abagale Floyd 15
Winna Floyd 13


United States Census, 1870, Louden, Whitley County, Kentucky
they are listed between Peter Tye McKeehan, Lydia Samantha Hart and Henry Brown.
John Buchanan, 32, Kentucky
Abigal Buchanan, 30 Kentucky
Nancy Buchanan 7 Kentucky
Elizabeth Buchanan 4 Kentucky
Jane Buchanan 1 Kentucky

1880 United States Federal Census
Name Nancy A. Buchanan
Age 33
Birth Year abt 1847
Birthplace Kentucky
Home in 1880 Davis, Caldwell, Missouri
Race White
Gender Female
Relation to Head of House Wife
Marital Status Married
Spouse's Name John Buchanan
Father's Birthplace North Carolina
Mother's Birthplace North Carolina
Occupation Keeping House
Household Members
Name Age
John Buchanan 39
Nancy A. Buchanan 33
Margaret Buchanan 17
Betty E. Buchanan 15
Nancy J. Buchanan 13
Mary A. (Alice) Buchanan 11 (married 1886 to George Riley Tye)
Wm. I. Buchanan 8
Sopha A. Buchanan 7
George-Dewey-Buchanan 6m