What diameter torsion bars?

IMO, spring/torsion bar selection is affected by several things.

• Intended use of the car.
• Weight supported by the spring
• Changes in spring rate at the other end of the car.

That said, I personnally would not go beyond a .890 TB unless you are also going to increase the spring rate in the rear or are not satisfied with the current handling balance.

Consider that stiffening the front suspension moves the handling balance more towards oversteer, while stiffening the rear suspension moves the balance more towards understeer. On the other hand increasing roll stiffness at the front increases understeer, increasing roll stiffness at the rear increases oversteer.

FWIW, what I did with the Demon was to install 1" TBs and six-leaf springs in the rear. I also added front and rear sway bars. For the rear bar, I fabricated brackets to replicate the body mount of the A37 police package of 1976. Bushings were replaced by polygraphite and the shocks are Monroe Gasmatics. Tires are 235/60-15 T/As. The car has a well-controlled ride, handling balance shows less understeer, and turn-in is much sharper.
Since I'm using radial tires, I went with a front end alignment that is better suited to take advantage of them. The most significant change was bumping up the caster setting to + 2º. I would have gone higher but that's all the body would give us.