Bring a large pot of water to a boil. Add salt and pasta, and cook according to package instructions until al dente. Drain, reserving 1/2 cup cooking water.
Heat olive oil in a large skillet over medium heat. Add garlic and red pepper flakes, and cook, stirring, for 30 seconds; do not let garlic brown. Stir in parsley, zest, 1/8 teaspoon salt, and a pinch of black pepper; cook for 30 seconds. Add wine, and simmer for 1 minute. Add clams; increase heat to medium-high, and cook, covered, until clams begin to open, 3 to 4 minutes. Stir in butter until incorporated. Add pasta to skillet, and toss to coat, adding reserved cooking water a tablespoon at a time to loosen, if desired. Transfer to a large serving bowl, and serve.