I have a 69 SS 396 restored (98 point car). Should I use a stock or afternarket rear sway bar?

stock ones are still available from GM, and are cheaper than aftermarket.

Also, only F41 cars had rear sway-bars (very rare in '69). To be "correct" you will also need boxed rear lower control arms, and a 1-1/8" front sway-bar.