Be sure to bring lots of variations on the buc bug. Green machines, smurfs, shady ladies in multiple sizes. Dark patterns like a black bear/green butt, conrad, or blue charm are necessary. Shrimp patterns like Alley's shrimp, tippet shrimp. or GP should be included. A brighter pattern like a cosseboom, green highlander or silver rat /silver doctor would round out the wet flies. Three flies you don't often hear about, but which I always take to the Miramichi are the salmon mickey finn, chief needahbah, and the bonehead. Dry flies should include some bombers and some Wulffs.
Of course, your other option is to ask your guide what you should bring or to call Doak's fly shop in Doaktown. Be prepared to be told that "all your green machines should have glitter tails" or something like that. Like all areas of the world, they always seem to have something new or different that it's claimed will work best.