When I drove to the Olds nationals last year I calculated roughly 16-17 mpg on the highway with my C headed 455 as well, not too shabby for a bbo with a 3 speed trans and 3.08 gears. Of course it doesn't have a wild cam and it has stock intake and exhaust manifolds. I started to use the argument that my carb fed Olds was better than most SUV's when criticized about fuel efficiency. Of course street driving is a different story
It actually gets better gas milage than my stock 72 350 daily driver (11mpg street, 15 hwy) with 2.14 gears and a three speed trans.