70 Duster Rear Drum Help

Welcome to the site. If your car hasn't been changed over you should have 5 on 4" brake drums. They are nearly impossible to find new unless you get lucky and find some on eBay or a similar site so good used seems to be the best option.

Here is a thread you might want to read.
http://www.forabodiesonly.com/mopar/showthread.php?t=255314