68 valiant /6 transmission question

If I have a 68 slant 6 car with a 3 speed auto transmission, will a Auto transmission from a 78 Aspen or volare work to replace the current transmission? What issues will I run into if I use the 78's torque converter and transmission? Or can I use the convert from the 68 with the 3 speed from the 78 aspen or volare?