They are different units internally, due to torque output and case if different. Look for 90-93 units. Non lockup with overdrive. You could go 94/95 too but they are lockup. 96 on needs an electronic trans controller. You can control the lockup and od with just two switches on the 95 back.There are some other ways to do that too. You may want to get onto cumminsforum.com or look for the "turbo diesel registry" which is a dodge/cummins publication. They have a forum also.