Look here for a spring rate comparison between different diameter torsion bars:
http://www.firmfeel.com/torsionb_a.htm
I have a slant six in my rag top running 0.940 bars. Slants and small blocks are close in weight, a big block comes in about 300 more pounds.
Chances are that your suspension has been bottoming out if your bars are weak giving the feel of a stiff ride. My Dart is softer riding than my 300 C AWD, so as things go, compared to a modern car, the Dart is on the soft side. I have a firm feel rebuilt steering gear from Steer N Gear, sway bar, and the car now feels and drives & corners flat like a modern car. With stock 0.810 bars it was a flopping, wallowing mess.
Where you have a big block I wouldnt hesitate using a 1.00or thicker bar, in conjunction with a good anti sway bar, and a Firm Feel steering gear; youll be happy. Also this would be a good time to install new front end bushings if yours are getting on the worn side.