Below are roughly what the factory offered in the better handling packages
for the e-bodies. These can serve as reference points.

'Cuda with rear sway bar:
0.90 T-bar (111 lbs/in)
0.94" front anti-sway bar
0.75" rear anti-sway bar
Heavy Duty rear springs with 4.5 Leaf (132 lbs/in )

Although non-rear sway bar package seems to have been more common:
0.90 T-bars
0.88 front anti-sway bar
heady duty rear leafs.

A slightly larger t-bar came on Hemi 'cudas and maybe other options.
A stiffer 150 pound/in rear spring came on Hemi and maybe other options
A .88 T-bar and 115 lbs/in leafs came on 2 barrel 383s

Someone who is into restoration will know exactly the options and what came with what. But this should give you some idea of what you're aiming for. Most important, if you stiffen the rear with more spring rate or rear sway bar, you definately want to increase the front t-bars and sway bar at the same time. You pretty much can't go wrong increasing the front t-bars to .90 -1". But if you're staying with smaller t-bars than that, be conservative on the front anti-sway bar.

edit: Fixed decimal places!! See AMX's post below