Heat 3 tablespoons oil in a deep 12-inch heavy skillet over moderately high heat until hot but not smoking, then saute pancetta, stirring occasionally, until golden, about 6 minutes. Add garlic and saute, stirring, until golden, about 30 seconds. Stir in clams, clam juice, wine, and chiles and boil, covered, until clams open, about 4 to 5 minutes (discard any clams that do not open after 6 minutes). If using whole chiles, remove with a slotted spoon and discard. While clams are cooking, cook pasta in a 6-quart pot of well-salted boiling water until al dente. Drain in a colander, then add to clams and simmer, stirring, 30 seconds. Toss pasta with parsley and drizzle with remaining 5 tablespoons oil.