Heat 3 tablespoons oil in a deep12-inch heavy skillet over moderately high heat until hot but not smoking, then sauté pancetta, stirring occasionally, until golden, about6 minutes. Add garlic and sauté, 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.