Cyril was a very distant cousin -- and yes, 3rd cousin, twice removed is
the way genealogy software describes the connection which is available from
family trees online:

Samuel Langhorne Clemens and Cyril Coniston Clemens are 3rd cousins 2 times
removed.  Their common ancestors are Ezekiel Clements (sic) (b. abt. 1680)
and Christina Castell.

To answer Martin's original question --
Chapter 54 of LIFE ON THE MISSISSIPPI and the entry for "Clint Levering" in
the Biographical directory in HUCK FINN AND TOM SAWYER AMONG THE INDIANS,
(UCal edition), p. 331-332 may be close to the source of Harnsberger's
account.

Barb