72 Duster New torsion bars what size?

1" bars or bigger. And if you enjoy corners at all, you should definitely add sway bars as well.

And 1.00", 1.03", and 1.06" bars aren't bad at all for ride quality either. The stock torsion bars on these cars are a joke, they're HORRIBLE for even just driving around town - no cornering stability, very little resistance to braking dive, and prone to massive understeer. I had a set of 1" Just Suspension bars on my Duster for awhile when it had a 318/904 in it. They were a huge improvement over stock, and they still rode very well. So well, in fact, that I swapped them out for a set of 1.12" bars. The 1" bars were still too soft for my plans.

With 1.12" torsion bars and sway bars front and rear, my Duster handles amazingly well for an old musclecar. The 275/35/18's in front and 295/35/18's out back help too. :D And with the Fox shocks, the ride quality is fairly similar to a modern sports car. I don't think my Duster rides any harsher than my g/f's 2013 Mustang with premium package suspension.