Much to my surprise, I found the article I wrote for the Walt Whitman
Encyclopedia on Twain and Whitman:

CLEMENS, SAMUEL LONGHORN [MARK TWAIN] (1835-1910)

Clemens, popular for his fiction written under the pseudonym "Mark Twain,"
and Whitman are often compared as vernacular writers of nineteenth century
American democracy. Clemens' Adventures of Huckleberry Finn (1885) is often
considered the literary companion piece to Leaves of Grass, both works
subjects of book bannings that were eventually hailed as turning points in
American literature.

Comparisons include the authors' similar backgrounds, time spent as
apprentice printers, their personas as self-made, rough-hewn artists, and
their sympathy with downtrodden peoples. Both championed American idioms and
speech and the individual against conformist society.

Yet, the two showed only perfunctory interest in each other. Whitman said
Twain "might have been something. He comes near being something: but he
never arrives." In turn, Twain noted "If I've become a Whitmanite I'm
sorry--I never read 40 lines of him in my life." This claim is probably
exaggeration; Clemens' personal copy of Leaves of Grass contains many
marginal comments by Clemens, and in 1897 Clemens'-owned Charles L. Webster
and Co published Selected Poems, by Walt Whitman with Whitman's special
permission.

Clemens provided financial support for Whitman on several occasions
including $100 for a horse and buggy and $200 for a cottage to "make the
splendid old soul comfortable." In 1889, Clemens sent Whitman a
complimentary copy of A Connecticut Yankee in King Arthur's Court.

In 1884, Clemens grouped Whitman with other writers in an anecdote, and he
attended Whitman's 1887 eulogy for Lincoln at Madison Square Theater. His
ambivalent feelings about Whitman were reflected on Whitman's seventieth
birthday when Clemens sent an impersonal, ambiguous telegram and in an
unfinished essay "The Walt Whitman Controversy" in which Clemens worried
about Leaves of Grass sexual frankness saying the book should not be read by
children.

Bibliography

Gribben, Alan. Mark Twain's Library: A Reconstruction. Boston:

G.K. Hall. 1980.

Kaplan, Justin. "Starting from Paumanok . . . and from Hannibal:

Whitman and Mark Twain." Confrontation. Vol. 27-28 (1984)

338-347.

Sorry for the lack of details--the article had a word count, but I do recall
finding information in Twain letters and notebooks. This was maybe 17 years
ago.