Stiffer shocks with rear leaf springs

I am against air shocks. If the springs are doing their job then you don't need air shocks. Shocks are really only to "control" the suspension. The springs are for the ride height.
I suggest a high quality pair of traditional style shocks.You are going to find a love em or hate em attitude toward air shocks. Good luck to you and your friend with the project.

P.S. You guys are lucky to have the 360 Darts from the late '70's. Is rust much of a problem for you?