When you accelerate the diff wants to send the tailshaft up though the floor.
The leaf springs transfer that same force to the car body.
Have a look under a Capri or German RS2000. They have the soft skinny leafs with a arm attached to the body, like GT Falcons did back then. We lucked out with the eskies.
Cal tracks? would be an idea, they are leaf spring supports for heavy loads.
$50 including postage for the rear sway bar?