Speedometer Trouble

I don't know what pics you've looked at but I know the speedometer end of the cable changed from screw on collector nut to a plastic clip in 1968. So yours is correct at that end. As for the trans end of the cable... That may have changed to a clip of some sort but... as far as I know only the size of that collector nut changed over the years.
I don't know of aftermarket instruments that will mount in the OEM panel or behind the OEM bezel so figure that into the total cost. Good luck with it.