I would think that the newer the better. There is a different counter shaft assy. part # for some 5600's that have a warranty problem and the first ones in the Ram trucks had a smaller input shaft ( 1-1/4" v/s 1-3/8" ). I have a 2002 six speed and it shifts great and I am VERY happy with it. Some others have had problems though so you may get some better responses on the subject. HTH, Thomas