You are about right on your RPM at 65 mph. I'd say maybe even a little higher 3300, depending on tire size. If we are right about the RPM you have a 4.10 rear axle.
You have two choices change the rear axle or install an over drive. As Hank says a T-5 is a good choice. There are a lot of folks using them and they are fairly common. It might not be the best choice if you are going to get a big engine and bang gears or haul heavy loads. Then the choices near a bit NV4500, MY6 (A833) are a couple, unless you want to consider an automatic.
