Google your favorite gear ratio calculator. You have an automatic so if you can plug in your gear ratio's and combine that with your rear end ratio and tire sizes you can get an idea of the RMP's you can expect at given speeds. I had a 3.70 ratio rear with my top loader 4 speed and it was good in town but bad on the highway, I did not like it. I swapped to a 410 stroker with 3.25 rear gears and am real happy with that.
Here's a couple of links that should help with the rear-end question and what gear ratio you want. As has been said, it depends if the stock rear-end is still on the car. The data tag on the rear-end should tell you more.