I'd consider replacing the cable - RockAuto doesn't have a B body list but the do show an E body listing - around $10

https://www.rockauto.com/en/catalog/plymouth,1970,cuda,7.2l+440cid+v8,1231759,interior,speedometer+cable,1440

I'd pull the cluster, remove the speedo and lube the pivot points for the needle on the speedo, any gears, and the flywheel assembly, with a paint brush very lightly dipped in wd 40. This is almost like jewelers work, lubing the insides of a watch. And I'd make sure the speedo cable interface on the back of the speedo isn't gummed up with 50 year old grease, or dry as a bone. It's real easy to ruin the face of a speedo with just the natural oils on your skin and fingers, never mind a lubricant, so judicious work is called for. I use a clean artists brush to dust off the face of gauges, so as not to smudge them. You can sort of bench test it by connecting the cable to a drill.