Rub meat on all sides with saltpeter and 2 tablespoons salt; place in a large plastic, glass, or stainless steel container with a tight-fitting lid.
Combine remaining 2 cups salt, molasses, allspice, cloves, and nutmeg, stirring well. Pour molasses mixture over meat; turn meat to evenly coat all sides. Cover and marinate 21 days in the refrigerator, turning meat each day.
Remove meat from marinade; discard marinade. Wrap securely in cheesecloth, and place in a large stock pot; cover completely with water. Bring to a boil. Reduce heat; cover and simmer 3 hours or until meat is tender.
Remove stock pot from heat; set aside. Allow meat to cool completely in cooking liquid in stock pot. Remove meat from cooking liquid, discarding cooking liquid. Chill overnight. Remove cheesecloth.
Transfer meat to a serving platter. Cut diagonally across the grain into thin slices to serve.