I believe YNP has it's own license which is a lot less than a Montana out of state license. I'm far from an expert but I never had much luck in the Park just casting some spinner baits, etc. I don't think live bait is allowed and there is a lot of fishing pressure where the rivers are easily accessable. I did catch some rainbow in the Yellowstone River not far from Gardiner, walking west down the river on the Park side. Might be better to hire a guide and fish outside the Park. Ask in the sporting stores in the towns.