Forty years of crud and abuse adds up. I disassemble the spring leaves, clean and inspect, replace bushes in both the springs and the shackle, paint, and reassemble. Spring leaves should have a uniform bend with no flats or reversals. Otherwise, they must be either re-arched or replaced. If a proper spring shop is not available re-arching can be done with a big hammer on an anvil . Springs should be lubricated. I prefer a dry lube or motor cycle chain wax as grease collects dirt. Oh yeah… rear springs are not symmetrical. A pattern assures everything goes back the right way.
Recently I looked at a set of later model springs and noted that they do not have the screws with spacers of the older style. I have no idea whether or not the new springs can be disassembled.