The Incredible 2-Headed Transplant(1971)

Two heads crafted on the body of a giant... The most fearsome living force ever created by man.

Dr. Roger Girard is a rich scientist conducting experiments on head transplantation. His caretaker has a son, Danny, who, although fully grown, has the mind of child. One day an escaped psycho-killer invades Girard's home, killing Danny's father before being gunned down himself. With the maniac dying and Danny deeply unsettled by his father's death, Dr. Girard decides to take the final step and transplant the killer's head onto Danny's body.