I've run highway (with my 727 tranny) using 4.30, 3.91, 3.55, 3.21, and a long time ago a 2.76. For today's more common higher mph (~70-80), I like the 3.23 as in my car now. a 3.55 brings it up a little, and the question becomes how long time-wise and at what mph are you willing to cruise? I like a typical 1-2-3 hour cruise at ~3200-3500 rpm max, ~70-75 mph. I'm planning to eventually run a 2.94 setup (will drop by ~500 rpm at hwy speeds).
Also, hopefully you have some tall diameter tires, such as 275-60-15 in the rear (28" diameter).... that helps a,little and with some good comfort.