Stock tune is horrible for highway cruising. I could not use OD and it felt like 50% throttle made the same power as 75% throttle. It seemed to lose power past 75% throttle. For some reason these engines did not have knock sensors. To ensure that the engine did not ping, they pulled tons of timing when loads goes up. With stock gears and 35s, even small hills would slow me below 70 mph. OD was not an option.
After altering the timing tables, I use OD on the interstate except in hilly areas. I can now accelerate up hills on the interstate that would slow me down. Tuning will not in anyway replace a gear change, but will help no matter what gears or tires you have. I have 5.38s to go in it. They should help a ton. 70 mph will be just under 2500 rpm, which like others have said, that is where the 4.0 wakes up.