Bars don't really wear out, they are either springs or they are not. If they haven't been damaged then they should still have the same spring that they did when new.

If the bars have been overloaded at some point and twisted then that is a different story. You could have something else that is moving around or some other sort of issue.

The SB bars are too small to control a BB car for normal street driving but they'll work okay for drag racing. For street driving you should use the BB or the Hemi bars. I have the 0.960 bars in my '65 Coronet and they work nice on the street with a high quality shock. It just all depends what you are trying to do with the car.