I added a bar to one of my trucks and was very pleased with how much it helped. It was a BB truck, so there was some weight up front moving in the corners. I did the poly bushings at the time of install also. My current truck had a bar on it already and I put poly bushings in it also and didn't notice a difference from just that.
I don't know maybe you were expecting to much, it is still an old pickup, not a slot car.
At least you were able to notice some kind of difference.