Shocks came to be with the torsion axle. And that was some where around 1962
The leaf spring you have have better dampening qualities that the torsion axle so yuo may not have much benefit from having shocks installed.
If you still want to put shocks on your ride you will have to fabricate or have fabricated mounts to install shocks. You need to calculate the max travel up and down the axle will have to get the correct shock. You want a shock that will extend further than the greatest distance the axle will move away and compress a little further than the axle will move up toward the trailer frame.
The new axle manufacturer may have a kit to buy that can be attached to the axle and frame.