Into the weeds engine design for fuel efficiency discussion.

.035" Quench on iron heads allow higher static compression ratios without detonation. As partially stated above, thin rings reduce friction and also measurably reduce heat load into the water jacket. An intake manifold with high velocity runners helps with better cylinder filling and swirl, and aid in scavenging. If you want to have a little fun get a cam with a little more overlap that has timing events that increase dynamic cylinder pressure if your static is a little low. Whiplash cams do this pretty well, as well as Thumper and Bootlegger cams. On a big block I recomment an original Torker. Even though it is a single plane, the long straight, relitively small cross section runners make a ton of torque on the dyno tests I have seen, and from my own experience they really kick butt on 303's and 440's. Also (on a big block) 2" long tube headers with 24" collector extensions and a crossover will really braoden out the torque curve not not affect higher RPM power. And if you are running a carburetor, highly recommend a Thermoquad. If you put a wideband O2 sensor in each collector you can tune them with metering rods, jets and springs to perform very close to fuel injection. If running fuel injection, highly recomment a throttle body solution becasue the older intakes and head ports are designed for "wet" flow, and becasue the additional cooling and evaporation rates help fuel economy, torque, and helps control detonation. In hot climates water/meth injection is very effective, even on a naturally aspirated engine. And if you ever have to take off a head, you will be astonished at how carbon free the combustion chambers, pistons and valves are. They look new after a ton on miles. Multi-spark is a great choice, and the facory hall-effect distributors work perfectly for street or strip. Don't forget back-cut valves, minimum spring weights for the cam, plunge cutting the bowls and blending, and to run the factory stamped steel rockers if your spings are singles with a damper and your lift is close to .5" or so. They are very light, and becasue of the design are cushioned by a hydrodynamic oil wedge just like the bearings, so almost no friction. Of course this assumes a hydraulic flat tappet cam.