My '69 Coronet R/T had the same issue. I got the parts for the 1970 B-Body which had self-adjusters on the 11" rear drum brakes.
You will need the star wheels, adjuster lever, adjuster lever spring, cables and cable guides.

I think these kits have what you need?

https://www.rockauto.com/en/catalog/dodge,1970,coronet,7.2l+440cid+v8,1496006,brake/wheel+hub,drum+brake+self+adjuster+repair+kit,1904