Bring a large pot of water just to boiling. Add chicken breast and reduce heat to a gentle simmer. Cook chicken in barely simmering water for 12 minutes. Remove to cutting board; return water to boiling; add a little salt to the water.
Add pasta to boiling water and cook as per package directions, 9 minutes. Add pepper during last minute of cook time. Drain.
Meanwhile, melt butter in a medium-size saucepan over medium heat. Whisk in flour, chili powder and salt. Add milk in a thin stream, whisking constantly. Bring to a simmer over medium-high heat; simmer 2 minutes. Remove from heat and whisk in shredded cheese.
With two forks or your hands, shred chicken into bite-size pieces. In large bowl, combine pasta and pepper, chicken, scallions and cheese sauce. Stir until blended; top with crushed chips and serve.