What diameter torsion bars?

I wouldn't hesitate to run 1" bars on your car. From the factory these cars were horribly under sprung in the front. They understeer like crazy. With a 383 in the front a 1" bar will be more than compliant. I have a couple pairs of Just Suspensions 1" bars, and all of my cars are small blocks.

My challenger, while a heavier car, has 1.12" torsion bars. It's quite a bit stiffer than the 1" a body bars, but its still not a harsh ride. Probably stiffer than most folks like, but still not bad as a daily driver. Should ride pretty close to a 1.06" a body bar.

As far as balance goes, I don't think you necessarily need to change the rear springs to run 1" bars. If you look at the handling recommendations from Mopar for the circle track cars, you'll see that they recommend a 120lb rear spring, which is SOFTER than stock. From the factory these cars were undersprung in the front and arguably oversprung in the back. 1" bars, stock leafs, and front and rear sway bars can balance the car pretty well. Not saying you can't do better than stock springs in the rear, but they're not as far off as you might think.