I have a '67 New Yorker that I plan to build for Interstate Highway cruising. It has a 440/727 trans, but I don't yet know which rear end gears; I'm hoping for 323 or better. The 440 provides plenty of torque and I want to increase it in the 2,000 RPM range.
These are ideas I will keep in mind for maximizing mileage:
Stiffer secondary springs for the Holley carb.
Maybe taller tires if gears are too low.
Better intake and exhaust.
I did some of this to my '68 GTX daily driver during the oil embargo in the early 1970s. I was getting 22 MPG on I-95 in the flat lands of GA, South Carolina, and North Carolina with the car loaded with all my belongings. Of course, driving at 60 MPH and being light on the gas pedal while driving around in towns (I-95 wasn't completed at the time) also helped.