THE INCREDIBLE TWO-HEADED TRANSPLANT (1971)
Directed by Anthony Lanza
Starring Bruce Dern and Pat Priest
Totally hilarious camp classic stars a young Dern, who had yet to break it big in Hollywood.
He plays a mad scientist who grafts the head of a sociopathic killer onto the body of a hulking retard. Casey Kaseum plays the hero!
I caught this movie late on the now-defunct San Francisco TV monster movie show "Creature Features" one night in the early 1980s with a group of other similarly disturbed kids and we loved it—especially the film's ludicrous theme song, "It's Incredible."
A must see—if you can find it.