From Mississippi? hmmm... come across and roll up through Taos into colorado and turn west... cross either Cumbres pass with a quick dip back into New Mexico and go up through Chama or over Wolf creek to get to Durango.
Run up through Silverton and Ouray to Montrose... work NW to Flaming Gorge then North on a fast run through Idaho and over Lost Trail pass into Montana and on to Missoula...
...or angle east and come up through Jackson Hole and out the NE exit of Yellowstone and over Bear Tooth... or west out through West yellowstone... into Idaho... or keep on N/NW to Missoula
Be sure to stop at the Harley Dealer on the west end of Missoula for a cup of coffee at the hut there. The coffee sucks but the Ladies are Awesome!
Then west over Lolo pass and down the Lochse river... through Idaho, some great roads there... with the Pacific just a couple days west... or... back NE to Glacier and roll over Goin' to the Sun road and then make the long roll across Montana to the Black Hills...
... and that, will scratch about a part of a sliver of 2% of what you've got to choose from... Confused?
you could just fill the tank... pick a direction... grab your handles and roll!