Whatever you run, raise the compression to 8.5:1 to run off regular unleaded and keep a mild cam in it, that will help a lot. Don't overlook the ignition, an electronic ignition or dual points kit will keep the timing and dwell in spec for longer and you won't have to tune it up as often. Maybe even have the advance curve modified if your engine can stand it. Ultimate would be to convert to fuel injection but that's a pretty big project.
Last time I posted my fuel economy numbers people told me I was lying so I won't, but with a dual point kit and a well sorted out Rochester B with a 261 in a half ton I was doing pretty well. Keep narrow, tall radial tires and it will steer nice too.
