4 speed auto into '65 Barracuda?

As mentioned, many issues to deal with - cut tranny tunnel and reweld, rebuild 4-spd tranny to use your small snout torque converter and input shaft (if possible), adapt your cable shifter to the tranny, custom driveshaft, ...

The easiest way to lower hwy crusing rpm is tires with a bigger OD (see Tire Rack website). Most people here go the other way to low gear ratios for 1/4 mile times, but I am with you. Before that, consider associated changes. If you have 13" wheels, bigger wheels will look better anyway. If you have drums and want front disks, you may need different wheels anyway. You can get affordable new 17" alloy wheels that fit your car for the "big wheel, thin tire" look that is popular ("Fossified").

Other things for better mileage include electronic ignition. See posts for a cheap HEI upgrade. EFI would help mileage and driveability. The cheapest is a used Holley Pro-jection setup (~$150), but they are a bit quirky. I have a plan for cheap MPFI for my 273 someday. Any EFI requires plumbing a high-pressure fuel pump (see my post).