What torsion bars and leafs on a 74 Duster?

Torsion bar size depends on what engine your running.
My Scamp is still running the stock torsion bars with the small-block.
My Duster with a 383 i'm gonna use .920 diameter bars.

Leaf springs i'm running 6 leafs in my Scamp and Super Stock are going into my Duster.
As for your car riding like a boat it just sounds like your shocks are shot!

And no one i know runs air-shocks any more,they make the cars ride to stiff! :)