With what you say here, you are best off with the 2.71's that you have now. Here in Nebraska, Interstate is 75mph with most running 80.. 3.23's even suck at those speeds without an OD at least. With that short of tire you probably even need 2.45's, real dog though. On a 300 mile jaunt on the freeway, you will be sorry you have anything else than what you have. Just my opinion. I have a 67 Dart with a 273 auto with 2.71 gears, a 71 Polara with a 440 auto and 2.76 gears and a 69 Coronet R/T with a 440 that has 3.23 gears, that I'm going to install 2.76 gears when it gets a new engine. Sometimes you have to give up the tire burning for a little sanity. I drive my cars on the Interstate regularly. I don't like everyone passing my old cars, and they don't! Good luck with your decision.