A sway bar would be a good addition. Most station wagons came with them. See if you can find a replacement bar for a '57 Belvedere wagon...
DON'T put a panhard bar on a leaf spring suspension. Entirely unnecessary (the springs locate the rear end) and all you will be doing is inducing bind into the suspension travel. The leaf springs, basically, go straight up and down during suspension travel. A panhard bar travels in an arch. If you put a panhard bar on that is too short or to long it will try to move the rear end from side to side as the suspension cycles... Not what you want it to do... Let the leafs do what they are designed to do.
Rebuilding the leaf spring pack with new bushings (polyurethane) and some teflon buttons between the leafs would add a lot of performance and a good set of shocks would get you a decent handling, safe, car.