They also had different cases. As was mentioned, the tail shaft was longer for the OD unit and it had a different mount set up for the frame but they were still closed drive set ups. The ones prior to the change to side shift had a different shifter set up and not all were overdrive type units but the input, cluster, and 2nd gears generally were interchangeable with Ford units. here is a link to the different cases.
http://www.vanpeltsales.com/FH_web/f...ed-options.htm