Andy, you might want to put a tape measure across the width of the leaf spring on the back end of that 56.

My original 49 still had the narrow 1 3/4" wide springs (a lot of them). If your going to upgrade the rear springs, buy the frame brackets and go with the wider modern springs, if Dodge still had the narrow springs in 56. Not sure what model year Mopar started using the wider leaf springs.