I have 1977 Camaro. It had a factory 305 but know has a 350. The speedometer cable melted when the floors were re-done. The problem is I cannot get the new speedometer cable to fit into the socket of the speedometer. Is there a difference in the diameter of the speedometer core between a 305 engine and a 350. It had a 350 trans. Hopefully this is enough info.
The engine size shouldn't matter,as long as you bought the cable for the right kind of transmission.You might want to have someone spin the inner cable (on the transmission end)as you are trying to mate the cable with the speedometer.This is how I did mine last time,because if I remember right the cable didn't want to go into the speedo easily.