Drain plums, reserving juice; pit and sieve plums. Set plums aside. Combine plum juice with lemon juice, soy sauce, Worcestershire sauce, basil, and garlic. Mix well; set aside.
Remove the fell (tissue-like covering) from roast with a sharp knife. Place roast on a rack in a shallow roasting pan; sprinkle surface of roast with salt and pepper. Insert meat thermometer, if desired.
Bake, uncovered, at 325° until desired degree of doneness: about 1 1/2 hours or 140° (rare); about 2 hours or 160° (medium); about 2 1/2 hours or 170° (well done). To glaze, baste with plum juice mixture frequently during cooking time. Reserve remaining plum juice mixture.
Transfer roast to a warm serving platter, reserving pan drippings. Let roast stand 10 minutes before slicing.
Combine reserved sieved plums and remaining plum juice mixture in a small saucepan. Simmer 5 minutes, stirring occasionally.
Garnish roast with parsley, if desired, and serve with plum sauce.