Lady Frankenstein 1971

From IMDb: When Dr. Frankenstein is killed by a monster he created, his daughter and his lab assistant Marshall continue his experiments. The two fall in love and attempt to transplant Marshall's brain in to the muscular body of a retarded servant Stephen, in order to prolong the aging Marshall's life. Meanwhile, the first monster seeks revenge on the grave robbers who sold the body parts used in its creation to Dr. Frankenstein. Soon it comes after Marshall and the doctor's daughter.