Re: Montana Trip Advice
It all depends on where your interests lie, and what sort of fishing you like to do.
For sheer variety, I'd do either Livingston, Bozeman, or West Yellowstone. All close to YNP, all have a variety of fishing situations (Big water, little water, freestones, spring creeks). Lots of big name water nearby- Yellowstone, Gallatin, Beaverhead, Jefferson, Madison.
Another good option would be Missoula, Butte, or Deer Lodge- lots of freestones, and some big water, too. Some of the major rivers are the Clark Fork, Rock Creek (THE Rock Creek), and Bitterroot- and Butte is fairly close to the Big Hole, too.
I'd suggest picking up John Holt's books on fly fishing in Montana. He covers both sides of the Divide in pretty good detail, and gives you a good idea of a number of rivers, creeks, and lakes, feasibility of access, species present, patterns, nearby towns- basically all the options which would be available to you. IMO opinion some of the best money you'll spend if you're trying to plan a trip up there.