His name is also spelled as McLain in many records, perhaps more than have it as McClain, but it is easier for me to keep track of all the McClain/McLain/McClane families in the Fleming/Morgan/Bath/Rowan county area if I spell them all the same.

He must be the William McLain in this death record, since there was no other William McLain of the same approximate age who lived all his life in Fleming Co., but it has his parents wrong:

Kentucky death certificate records William McLain, age 87 years 9 months 26 days, born 11 June 1837 in Fleming Co., Ky, died 14 Apr 1925 in Fleming Co., Ky, father Howard McLain, born in Fleming Co., mother’s maiden name Mary Eden, born in Bath Co., cause of death lobar pneumonia, informant Edger Cooper, Ringos Mills, Ky.

I cannot find William in the 1860 census.

1870 census of Fleming Co., Ky, Hillsboro precinct, enumerated 11 Aug, lists on p. 33 as family #226/223:
Skinner, Phenis, 66, farmer; Mary, 66; Downey, Henry, 17; Lee M., 14; McClain, William, 32, shoemaker; Louisa, 28; Mary, 4. All born in Ky.