Tthe 2 shafts are dimensionally the same in diameter .. they have to be since both trans use the bearing, they are different in length though , maybe 1/4 - 3/8 " , the dimension of the fit is determined by the size of the hole in the case.

To correctly assemble and disassemble the OD trans the cluster pin needs to be removed and the cluster gear is sitting in the bottom of the case, install the main gear assembly in the trans , rotate the tailhousing so you have access to the cluster pin hole , lift the cluster into place with the thrust washers in place and slide the cluster pin into the trans. the cluster pin seats and unseats from it's bore on the front of the trans with very little if any force.

The OD gears can be installed in a small bearing case and I'm not sure if there is the large OD retainer that fits this case or not , but a standard 833 23 spline retainer will work and then you wouldn't need the OD bellhousing .