My K5 is still pretty much stock. Quadrajet on a 305, 700r4, 3.73 gears and 31's with no lift. Still have the air dam, and the bug deflector my grandfather installed when he bought the truck new in '84.
I usually think I'm doing pretty good to get double digit mpg around town. 7 to 9 is more common than 10-11, but I also cane the shit out of that 305 to get it moving at an acceptable pace. I'd probably see the 10-11 more often if my right foot wasn't so heavy, or if I ran with AC on less frequently.
On the highway, 17 is pretty common, but I almost never see better than 18, except for one freak incident driving through Rocky Mountain National Park where I somehow got 21 mpg.