You do know you are going to have to change you speed-o-gears to match the ratio in the rearend. But I do not know where to look for those.
If the truck is running now, you can check the accuracy of the speedo/ odometer with a gps. I would personally pay more attention to the distance traveled vs speed as a 65 year old speedometer probably won't give the right speed anyway. If the odometer is fairly accurate you can compute a ratio for a odometer corrector with the old axle and the new axle and tires. A speedometer corrector usually attaches between the speedometer cable and the transmission.
