78 Aspen 904 tranny into 69 dart spedo cable issue

Your '69 Dart should have a separate speedo gear housing that has threads for the attaching nut on the speedo cable housing. The '78 speedo cable and speedo gear holder is all one piece. To use the later transmission, just pull the speedo housing out of your old '69 transmission and mount it in place of the speedo cable assy from the '78 transmission. You would want to use your old speedo gear anyway so your speedo stays accurate.