Rear gear calculation

I'd say the real question is are you really doing 70mph? The rpm seems high, but if the speedo gear is off because the transmission was ordered in a different car, or with a different rear ratio in that car, then the speed will be higher or lower. If the car is a sure grip, jack the back end up by the meatball, mark the driveshaft and tire, and turn the tire, counting the driveshaft rotations.