Burns finished at 51.28% in 2000 (about 7.7% lower than Bush in 2000). That translated into 2700 more votes than the Dem.
http://www.co.yellowstone.mt.gov/elections/results/ares1100/
Bush did get about 2.7% more of the vote in 2004 (61.7%) than in 2000 (59%). And the voters totaled 8k higher in 2004 than in 2000.
http://www.co.yellowstone.mt.gov/elections/results/ARES1104/default.htm
So the +7,100 estimate you have for 2006 may be high, but even if it is cut in half and the other counties break close to what you have Burns should still come out on top...in a squeaker.
Note: The stats in my post above are for Yellowstone county only (per the links).