Okay, I've been doing some more research on the internet, and the following is my official gotta see list.
Rocky Horror Picture Show (1975)
The Good, The Bad, and The Ugly (1966)
Dawn Of The Dead (1978)
Shaft (1971)
The Last House On The Left (1972)
Also, I've gotten a lot of good suggestions since I first posted, so I am planning to make a second list, starting with Burgomaster's suggestions. Please keep posting ideas, not just horror but any kind of 70's exploitation.
You haven't seen THE ROCKY HORROR PICTURE SHOW? The others I copied from your list are must-see, but THE LAST HOUSE ON THE LEFT, a very important film, is extremely offensive and a film I loathe (yet I cannot deny its enormous importance). You should skip CANNIBAL FEROX; it's revolting and stupid.
So, you're looking for 70s schlock and SciFi?
How about FANTASTIC PLANET (1973 aka La Plančte Sauvage, French animation)? Truly "fantastic!"
DEAD PEOPLE (1973)? Scary! "No one will hear you when you scream!"
LET'S SCARE JESSICA TO DEATH (1971) Weird!
GRAVE OF THE VAMPIRE (1974) stupid and fun!
COUNT YORGA, VAMPIRE (1970)? Super duper dopey!
THE DARK (1979)? Oo-oo those eyes!