If you need to shorten it, it would just depend on how much you lower it and how far off center your rear axle ends up. I don't think I have ever had to shorten one because of lowering. I have lowered several 67-72 pickups without shortening the bar. It basically just ended up a similar angle in the opposite direction to where the rear end stayed centered. I lowered the rear about 6" on all the trucks I did and ran very little tire clearance, so if it wasn't centered, I would have known it.
