I have built and raced road race and circle track cars and found that puting a panhard bar on a leaf spring car did not have any appreciable difference given we I using poly bushings. This is with slicks, if it is a street car with street tires the tires would be the weak link as far as handling. If you do use a panhard bar it is best to go all the way over to the other side of the rearend from the frame to make it as long as possible. If the bar is short as the suspension moves up and down the bar could cause binding being the angle changes will be greater with a short bar as opposed to a long bar.
Chuck