It was the November 1992 issue of GQ, but the Huck-and-Tom
caricature was not on the cover. That issue's cover was a color
photograph of Bill Clinton and Al Gore, with the accompanying title:
"Huck and Tom: Gore Vidal Punches the Ticket." You can glimpse that
cover at this web site:
http://www.mediabistro.com/fishbowlDC/magazines/obama_first_pol_to_grace_gq_cover_in_15_years_64920.asp
    The color caricature of Clinton and Gore as Huck and Tom aboard a
raft was on the inside, illustrating Vidal's article, which trashed the
Democratic ticket.