You might discover the precise distant relationship in his self-published
THE CLEMENS FAMILY CHRONOLOGY 1610-1912 (1914). I have the original
pamphlet, which is scarce and too fragile to copy (or I'd happily do so for
you), but photocopy editions and CDs are available online at the usual book
sites. You'd probably want to double-check anything claimed in that booklet
with records available at ancestry.com.  Clemens also published a second
work on Twain's geneaology in a magazine, BIBLIO, volume II:1-12 (July
1922 --June 1923). Twain called Will Clemens a "troublesome cuss" because of
Will's other publications exploiting Twain, which are well-known.

Kevin Mac Donnell
Austin TX