I believe you may be mistaken about the lifespan of William S. Summitt. The William Summitt born in Spencer County, Kentucky, would most likely be the father of the man that died in 1976. His father's name was William J. Summitt, and he can be found at the age of three months on the 1850 Federal Census for Spencer County, Kentucky. William J. Summitt's father and head of household in this 1850 census was also named William Summitt (born 1818) and his mother was Sarah Miller (born 1819).
William S. Summitt was the first born son of this William J. Summitt and his wife Amanda Melvina Morris. The 1900 Federal Census for Louisville, Jefferson County, Kentucky, shows William J. Summit as head of household with wife Amanda and eleven living children. William, Jr., is 20 years of age, and the other children listed are John W., Laurence, Rosa Lee, Lilly, Alice J., Ernest O., Maud, Oscar, Thomas Shouse, and Elizabeth "Lizzie" G. The 1910 Federal Census for Louisville, Kentucky, lists the birth of an additional child in 1900, Ida Mary.
A death certificate is listed for William J. Summitt in the year 1915 in Louisville, Jefferson County, Kentucky.
The Social Security Administration Index lists William S. Summitt's date of birth as March 4, 1881, and date of death as September 26, 1976. He and his wife Ora are buried in Eastern Cemetery, Louisville, Kentucky, and their memorials can be found on the Findagrave.com website under Eastern Cemetery. It would appear that there were at least three generations of namesakes for William Summitt consecutively in this family.
Best wishes on your continued research,