Torsion Bars

As pointed out, if you had an AC /6 it may already have the small V8 bars.

But, they were horrible too, all of these cars were drastically undersprung from the factory. I have 1" Just Suspension bars in my Duster. They really aren't that stiff. I'll probably upgrade them at some point to 1.03's or 1.06's.

1" bars are a good choice for most folks though, they'll dramatically improve your handling without giving a harsh ride, just something a little more modern feeling. And you don't have to to all the upgrades at once, but it would be a good idea to upgrade the bars at some point.