A third member out of a 55 and later will bolt in and it will be a 3.90. A 4 speed out of a 55 and later half ton will be open drive shaft and it will have a speedo gear for 3.90. 3/4 ton used a closed drive shaft to the center bearing and open from there and it will have speedo gears for 4.56 on 7.00 x 15 tires. A 4 speed from a larger truck will have a parking brake on the rear of the trans and will probably have the wrong speedo gears. I never worry about speedos. I just drive them as fast as they will go. I think there is a good chance that your speedo gear you have now would work. There is not that much difference in the rear of a SM420 with or without open drive shaft.