Driveshaft vibration

I have an unpleasant vibration in my 57 Buick Super starting around 50 mph. Following instructions in my original Buick shop manual I have definetely proven the vibration is in the driveshaft. I will have to remove the driveshaft, have it balanced and on reinstallation make sure it is aligned correctly with the splines at the transmission output. The problem is that this car has a torque-tube which means the work is labor-intensive. This is a get-it-right-firstime situation. How can I be absolutely sure the vibration will be gone? I cannot face putting it all together just to find a vibration still exists.