If you are shortening the driveshaft because you are swapping out the engine and third member to, lets say a 340/360 and an 8-3/4 you should have a new one built. The driveshafts that came with /6 and 318s with the 7-1/4 rear axle are weak. You can also upsize the yokes to 7290s. They will use a thicker wall tubing and you can drive(read burnout) with confidence.
