I don't have any specific information on this William. There is a William Capehart b. 1866 listed in the 1870 census with his mother Leanda Capehart and his brother Zachariah. That might be your William since he is close to the same age. I am not sure who his father would have been. It might have been William Capehart, son of John H. Capehart (AL), or it might have been Zachariah Capehart, son of Thomas Capehart (SC). Unfortunately the census records don't often give middle names or initials. Since Thomas and his son Zachariah lived in Polk County, I would think that would be a very good possibility. Have you checked marriage records in Polk County?