Go big with Kaufman Stone or Stimulator. I tie my stones for the Tom in size 6 1x long and my Stimulators size 4 2x long. Yes that is big but so are the bows that come out for the stone hatch. I have to order my #4 2x long dry fly hooks from back east because local suppliers don't stock them.
I have sold lots of my patterns to many travelers who wander into The Bighorn Trading Post, looking to buy flies that work. I love the looks I get when I show them those big Stimulators. I always tell them go use it, if it doesn't work bring it back and I will refund your money. Never had to make a refund yet.
As for the fish! There is a reason that the first time slot I book for my annual vacation is the first two weeks of August for the Stonefly hatch on the Tom. I don't tell how many I bring to hand or how large they are but I will say most are push over 20 inches.
Other flies that work well are Grasshopers #6, red bodied Tom Thumd #6, Irresistible #8, Chernobyl Ant #6.