The 292 was designed to compete with the 300 cubic inch Ford inline six- - - -both of them made for use in medium-duty box vans, school buses, and other gas guzzlers. They spent most of the time lugged down and pulling hard, at or near their rated top RPM and horsepower ranges. Getting good gas mileage from either engine is about as likely as finding a chicken with lips. I'd suggest gearing for a cruise RPM in top gear of around 2100-2300 RPM or so, which is right in the middle of ther engine's torque band. That will allow for a light throttle opening, especially with a real carburetor such as a Carter YF. A Weber makes a pretty good sinker when bottom fishing for BIG catfish.
Jerry