Remove and install new transmission 904 auto

I replaced the slant 6 and transmission with a 318/904 transmission in a 73 Duster. However, I cannot figure out how to install the speedometer cable. I have both cables run but is there a part I need to ensure the old cable will connect properly and provide accurate speed readings? I have been told by several individualsthat I need a special part at the transmission connection but none of them could tell me what part.