First make the stock: remove shells and heads, if available, from shrimp. Put heads and shells, or just shells, into a large saucepan, and add 5 cups water. Bring to boil, then reduce heat and simmer half hour. Salt and pepper to taste. Remove from heat and strain. Set aside 3.5 cups of this broth. Now heat the oil in the large saucepan. When just smoking, add chopped garlic and ginger. Stirfry one minute, or until golden. Add pork, deveined shrimp and sprouts; stirfry 3 minutes. Pour the reserved broth back in and return to boil. Stir in noodles, cook 5 minutes or until done. Transfer to a large serving bowl and garnish with the crab, cucumber and scallions; serve piping hot.