Another possibility that I haven't tried on a six is to recurve the ignition advance to give more advance at speed, igniting the fuel earlier, allowing a more complete burn and somewhat better milage. On my 350 it made a 10% improvement in mpg. I know the old hot-rodders did it, but don't know what it would take for new weights and springs on the six.
The rear gear swap is the most bang for your buck, you probably have a 5.14 or so gear now, going to a 4.10 would be a 20% difference in rpm.
