need some help for a newbie.

If I'm not mistaken the OD trans has a larger boss that centers the trans in the bell housing. Other than that they look identical. I think it was bigger in those for output shaft bearing support.

I had an Aspen 4 door /6 OD 4 speed with 3.21 rear gears. Had a Miser D100 short box /6 OD 3.55 rear. I couldn't shift in to OD at anything under 60 MPH! The Aspen got 31MPG driving it 35 miles each day to and from work.