The vast majority of leaf spring cars do not need rear sway bars. What sway bars actually do is increase weight transfer by lifting the inside wheel, which is exactly what you don't want in a rear-wheel drive car. Only when the body roll is excessive, should you install one. If you have bad understeer, and you install a sway bar, you just lower the rear end's handling ability to match the front. You're probably best off performing a Shelby drop with improved caster. That will help raise the front end's handling ability to match the rear.
Still, if you do want a rear sway bar, I'd stay away from the one that mounts the bar on top of the axle. It seriously compromises exhaust clearance, and I've had my exhaust torn out by it on a couple occasions. Get the adjustable Shelby style one that mounts the bar to the chassis, then you can fine-tune it to your car. Or there's this TCP sway bar that mounts under the axle so it won't interfere with the exhaust.
Still, if you do want a rear sway bar, I'd stay away from the one that mounts the bar on top of the axle. It seriously compromises exhaust clearance, and I've had my exhaust torn out by it on a couple occasions. Get the adjustable Shelby style one that mounts the bar to the chassis, then you can fine-tune it to your car. Or there's this TCP sway bar that mounts under the axle so it won't interfere with the exhaust.