speedo cable 65/66 Dart 4-speed question

You should be able to replace just the shaft (cable) and use your original housing. The 1965 parts book list the shaft as part number 2426164. Same part number for all 1963-65 A bodies with 6 or 8 cylinder, manual or auto trans.
You used to be able to buy cut to length shaft kits and make your own. Don't know if these are sold anymore.