what size torsions for a 69 dart for a little better handling?

Anything bigger than 340 bars would be good. I've run .890" (383), .990" (MP), and 1.14" (MP) in different cars. Both the .890" and .990" bars were run with a factory 383 sway bar and worked well. We also ran the .990" and 1.14" with a 1.25" front sway bar and .75" rear bar. The two sets of bigger bars were run with KYB shocks (known for being harsh) and the ride was not objectionable, in my opinion.

I've put thousands of miles on the 383 bars (torsion and sway) in both my old Dart hardtop and my son's Dart convertible. Both of those cars had standard modern gas shocks. Great road manners with a good ride in both cars (440/4sp drive train).