s6 to V-8 do I need to change torsion bars

You don't have to upgrade them, your car won't burst into flames or anything if you don't.

On the flip side, I'm with Jim. I had /6 bars in my '74 Duster, and they were horrific even with the /6 in it.

These cars were undersprung from the factory. Even cars with the factory bars that are correct for the engine they're equipped with don't handle well, bottom out the suspension, etc. And keep in mind the original bars are 40+ years old. If you're at all used to the handling and ride quality of more modern cars, you should put larger bars on the car. Larger than even the factory V8 bars.

Firm Feel, PST, and Just Suspension all make torsion bars for A-body's like your Duster. My recommendation would be to get a set of bars close to 1" in diameter if you're building a car for the street. The 1" to 1.03" bars offered by those manufacturers are pretty good for the street, they'll significantly improve the handling abilities of your car without rattling your eyeballs out of your head if they're matched to a decent set of shocks.