Speedometer cable

I am replacing the speedometer cable on a 1970 Duster. The replacement cable has a smaller threaded end than the original cable, so it will not screw onto the transmission. The car originally had a 904 trans, but was converted to a 727 trans several years ago. The auto parts store says that the cable is the same part number for a car with a 318 and 904 trans, or a car with a 340 and a 727 trans. Can anyone explain why there are two sizes, so I can try to find a cable that will fit?
Thanks