Put the dried porcini in a bowl, cover with water, and soak for at least 15 minutes.
While the porcini are soaking, rinse the clams in several changes of cold water until you do not see any sand at the bottom of the bowl. peel the tomatoes and cut into ½-inch dice. Peel and finely chop the garlic. Finely chop enough of the parsley leaves to measure 1tablespoon.
Fill a pot for the pasta with at least 6quarts water, place over high heat, and bring to a boil.
Lift the porcini out of the water and squeeze the excess back into the bowl; do not discard the water. Rinse the mushrooms under running water, then coarsely chop them.
Put the olive oil, garlic, parsley, and crushed red pepper in a 12-inch skillet or a 10-inch sautéuse and place over medium-high heat. Once the garlic begins to sizzle, add the porcini. Stir well, then add the wine and cook for 1 to 2 minutes to evaporate the alcohol. Add the porcini water. Be aware that there may be some sand at the bottom of the bowl, so pour carefully or strain through a paper towel. Cook until all the liquid is evaporated, then add the tomatoes. Season with salt and cook until any liquid the tomatoes release is evaporated, about 10 minutes.
Add the clams and cover the pan. Cook until all the clams open, 3 to 5 minutes. Don’t be concerned if some clams take longer to open; they are the freshest ones. Remove the clams from the pan and set aside. Continue cooking the sauce until most of the liquid the clams released is evaporated. When the clams are cool enough to handle, take the clams out of the shell and cut them into 3 or 4 pieces, about the size of a pea. Return the clams to the pan, stir for about 1 minute, and remove from the heat.
Add about 2tablespoons salt to the boiling water, put in the spaghetti, and stir until all the strands are submerged. Cook until al dente, then drain well. Toss the pasta with the sauce and serve at once.
You can make the sauce up to 2 hours ahead of time but wait to add the clams until you are ready to cook the pasta. Do not refrigerate.