In a medium bowl, sprinkle the cucumbers with some salt; toss to coat. Let sit until juices form, about 10 minutes.
Meanwhile, in another medium bowl, combine the chicken, garlic, ginger, Szechuan peppercorns and sriracha. In a large, deep skillet, heat the oil, two turns of the pan, over medium-high. Add the chicken mixture and cook, breaking up with a spoon, until browned and cooked through, 8 to 10 minutes. Add the stock, 1 cup water, soy sauce and fish sauce. Stir, scraping up any browned bits. Bring to a low simmer, add the chard and scallions and stir until chard is just wilted, about 2 minutes.
Meanwhile, in a large pot, cook the noodles according to package directions. Divide among bowls. Top with the chicken mixture and the cucumbers. Serve with the lime wedges.