I honestly wouldn’t run anything less than 1” bars on anything. The original torsion bars were sized for bias ply tires. Even if you run radials in the same size as the factory tires you have more grip and need larger bars. Even when I just had 225/60/15’s on my Duster it was undersprung with 1” bars and no sway bar.
Bergman Autocraft has great choices for torsion bars. The PST 1.03’s are popular because of their pricing and they’re a good size for a street car. Firm Feel and Sway Away make good bars as well in various sizes.
All of the original bars are like 45+ years old at this point. While they may be ok, they were never intended to be in service that long. They do fail, and they can fail catastrophically. For a car that will see frequent street use I think they should be on the list of stuff that gets replaced. Firm Feel offers a reconditioning service, they shot peen and re-heat treat the origin bars. That would be useful for someone doing an OE restoration, but otherwise I’d just replace them.
Nope. The spring rate is a constant tied to the metallurgy of the torsion bar. If you don’t heat the bar up red hot the spring rate does not change.
They do “sag” in a sense, the car will lower over time and to maintain ride height you have to turn the adjusters. That does not change the spring rate of the bar.