Bruce Dern, Casey Kasem, an altercation with a biker gang, Pat Priest aka Marilyn from the Munsters and a whole lot more early seventies atmosphere are all over this schlock-science horror feature from 1971. This movie takes an "Of Mice and Men"esque scenario on a sideways skewed trip into b-movies as the plot follows the grafting together of a criminally insane psychopath with a mentally challenged, but warm hearted fellow.
Anthony M. Lanza was the director. Aside from the The Incredible Two-Headed Transplant, the only other directing credit of his I'm familiar with is the 1968 biker movie The Glory Stompers. The soundtrack of 70s groovy, light rock is another "jewel" in the crown of this film and I continue searching the soundtrack sections at local record shops for a decent copy.