Well McCain actually has a more conservative voting record.
According to whom?
During the eight years that Thompson and McCain served together, they cast votes on 102 CQ-defined key votes and agreed on 83 of them - or 81.4 percent of the time. They disagreed 18.6% of the time, that’s pretty high considering that both are Republicans and both are considered relatively conservative. Just as an example from one year, among the instances in which Thompson and McCain differed were votes in 2002 to effectively extend a repeal of the estate tax beyond 2010, to authorize oil drilling in Alaska’s Arctic National Wildlife Refuge, and to postpone tougher automobile fuel efficiency standards. Thompson voted “aye” and McCain voted “no” in all three cases.
Some key SenateMatch responses for Fred Thompson and John McCain:
“Abortion is a woman’s right”
Fred: Strongly Opposes (voted to ban partial-birth abortion)
McCain: Opposes (voted to allow partial-birth abortion)
“Sexual orientation protected by civil rights laws”
Fred: Strongly Opposes
McCain: Favors
“More federal funding for health coverage”
Fred: Opposes
McCain: Favors
“Replace coal & oil with alternatives”
Fred: Opposes (Voted to preserve budget for ANWR oil drilling)
McCain: Favors (Voted to kill budget for ANWR oil drilling
“Allow churches to provide welfare services”
Fred: Favors
McCain: Opposes
http://www.ontheissues.org/senate/Fred_Thompson.htm
http://www.ontheissues.org/Senate/John_McCain_VoteMatch.htm
He most certainly doesn't have a more conservative voting record than Thompson. Thompson has the higher ACU rating.