I have a 1964 Valiant 170/3 speed/7 1/4, that was built for fuel economy. It will get upper 20's mpg. Problem I have is too much compression and too little cam. It is true 9.5 CR, and unless I have the timing "backed" up it pings. I need to work the distributer some more.
PS: I do not believe any factory stock slant six car got "real world" MPG of over 30 MPG. Upper 20's maybe.