Are /6 torsion bars the same as 318?

In most cases, no, cars with /6's have smaller bars than cars that came with 318's. Cars with A/C were usually equipped with the next size up though, so a /6 with A/C would get the smaller 318 bars while a 318 with A/C would often get 340 bars.

You don't NEED to get bigger torsion bars. Of course, your car doesn't have to handle well either. And your intended use of the car is very important in making any choices in setting up your suspension.

If this is a street/strip car with emphasis on strip, leave 'em. It'll suck in the corners, but weight transfer at the track will be decent. If it's a street car or regular driver, buy a set of 1" torsion bars (minimum) and join the current century with regard to handling. If you prefer well mannered handling and modern ride quality, go at least 1.03". If you're thinking about any kind of autoX or road courses, go at least 1.06", probably more like 1.12".

Handling with /6 bars is laughable, even in a /6 equipped car. Remember the stock spring rates were designed for bias ply tires and very little traction. The 1" torsion bars from Just Suspension are still pretty soft even with just a small block, but they're light years ahead of anything stock in regard to handling. It won't carve any corners, but if you're worried about the ride being too stiff they're a good choice because they ride soft. At least they did on my Duster. So soft, in fact, that I got rid of them. My car now has a 340 and 1.12" torsion bars with aftermarket sway bars front and rear, and it still isn't "too stiff" for the street. I use it as my primary transportation, and drive it almost everyday. Larger bars like that will need better shocks though.

The 1" bars from Just Suspension, and the 1.03" bars from PST are also less expensive than any of the Mopar Performance torsion bars in the stock sizes. Only way to go cheaper is to buy used or leave the stockers in there. And remember, in either of those cases your probably talking about using torsion bars that are 40+ years old with an unknown history.