Cook pasta in well-salted boiling water in a large stockpot according to package instructions, until just shy of al dente. Drain, cover, and set aside.
Season steak cubes with salt and pepper, and toss with 1 tablespoon flour. Heat 2 tablespoons butter in a deep skillet over mediumhigh heat, and fry meat until browned. Deglaze pan with Guinness and simmer, uncovered, 5 minutes. Reduce heat to low and cook, covered, 20 minutes.
Preheat oven to 350°F. While steak simmers, prepare cheese sauce. Melt 6 tablespoons butter in a medium saucepan over medium-high heat until bubbling. Whisk in remaining ½ cup flour. Reduce heat to medium and cook, stirring, 5 minutes or until roux is pale blonde.
Slowly pour in milk, whisking constantly. Run a rubber spatula around saucepan edges to remove any flour clumps. Bring to a gentle simmer, and cook 10 minutes or until thickened. Add 4½ cups Cotswold cheese, kosher salt, and ½ teaspoon pepper. Strain liquid from steak into sauce and simmer gently 5 to 10 minutes.
Combine steak, pasta, and sauce. Pour into lightly greased 3-quart casserole dish. Top with remaining Cotswold and Stilton.
Melt 3 tablespoons butter. Toss with bread cubes, then scatter over steak and pasta mixture. Bake 30 minutes or until cheese is bubbly. Let cool 5 to 10 minutes before serving.