O/D transmissions throw an entirely new factor into the equation.
With the .68 final drive in that TKO 5-speed (rear tire size & cruising speed dependant) 4.10:1 and even 4.56:1 would work well on the highway.
1:1 final drive + 3.91:1 gears + 27.1" tires (255/60R-15) = ~2,900 RPM @ 60 MPH
.68 final drive + 3.91:1 gears + 27.1" tires (255/60R-15) = ~2,000 RPM @ 60 MPH
.68 final drive + 4.30:1 gears + 27.1" tires (255/60R-15) = ~2,175 RPM @ 60 MPH
.68 final drive + 4.56:1 gears + 27.1" tires (255/60R-15) = ~2,300 RPM @ 60 MPH
1:1 final drive + 3.23:1 gears + 27.1" tires (255/60R-15) = ~2,400 RPM @ 60 MPH
1:1 final drive + 3.55:1 gears + 27.1" tires (255/60R-15) = ~2,640 RPM @ 60 MPH