3.23 to 3.55

I would not go to a deeper gear for Hwy. driving Tim. Your going to feel it in a minor way. If you do gears and a convert at the same time, you'll feel alittle improvment.
If you got it where it counts, a overdrive trans would be very good to install. Then you could go to a 3.90 or 4.10 without any major loss of Hwy. abiltys.

The ratio of the overdrive is .69.
That works like this;
3.90 X .69 = 2.69
4.10 X .69 = 2.82
4.30 X .69 = 2.96

Inconjunction with the higher stall (Need new lock up converter possibly) and gear change, it should feel preety good with the abilty to go into overdrive for the Hwy. without the hi-revin hassle.