Shocks will make no appreciable difference with the stock suspension.
Truck design back then was just the opposite of what it is today. Then it was stiff springs and flexible frames.
The springs don't even start to deflect until you have 2-3000 lbs on the bed. The spring travel, in any load condition, is so small there is nothing for the shock to dampen.
If you are running empty most of the time, reduce the air pressure in the tires.
Or you could strap a couple thousand lbs of something on the back to get the springs to start moving.
