The transmission, and rear end choice will have a lot to do with the answer to your question.
If you have a modern automatic OD trans or a 5/6 speed manual, then you have more options
If you are running a 727, or 4 speed, and want ANY fuel economy, IMO a stock(ish) 383 is the way to go.
I have pumped up quite a bit of 383's and 440's and in my experience they preform GREAT, but they are not good on fuel.
The most fun combo for the street I ever had,(and got good MPG) was a 383, with all the regular go fast goodies: headers, good duel exhaust with X pipe and flowmasters, carb, intake, electronic ignition, aluminum WP and housing, and the stock cam, with 3.23 gears. I got 15+MPG and it would surprise most people.
It wasn't the fastest thing on the road, but it may have been the best on fuel. of my big blocks.