Quickor and AddCo both make rear sway bars for early & late A bodies.
I installed a Addco rear bar, the 'street' (smaller dia.) on my '68 Dart over 20 years ago.
The entire setup has lasted and performed well for me.
The bar bolts to the axle with brackets and you drill and bolt brackets to the frame for the bar-end attachments.
I will get the heavier version for the convert.
Bill has worked on the inboard spring thing, but others have solved it by using a bar designed for another car or relocating the aftermarket bar to the top of the rear and. I would even look at the 73-76 front sway bar as a possible rear bar in a relocated spring car. I had one, but sold it for an attempt to run as a front bar on an early car.