This is from the dodgeram.org website:
If you have a 13" clutch, there will be a 3/4" aluminum spacer between the rear housing and the starter. If you have a 12. 25" clutch, the starter will bolt right up to the block. All DEE's manufactured after 8/99 had 1 3/8"x10 spline, and the bigger clutch set. Older 6 speeds had 1 1/4" and smaller clutch and on top of the transmission will be "DEA" in black marker.
Hope this helps,
John