it's not unheard of to have one leaf spring sag more than the other over time, they are not always tempered the same way and at the same time. and the fact that the drivers side is sagging more may be because most of the time, there is only the driver in the car. so more time with more weight on one side can do it. you would be best to do some research and see what the factory ride height is supposed to be, it seems to me that the rear ends on that era cars was lower right from the factory. if so, getting new springs probably won't help the sag much because they should be built to give the factory stance, you may have to use some longer shackles to get a more agressive rake to the car.
"The Government cannot give anything to anyone that it first doesn't take from someone else"......