The only difference that I am aware of between the cases is the material they are made of and the size of the bearing retainer. There were cast iron case OD units made. I have one.
Don't try to mix and match cluster gear shafts unless something is broke, keep the gearset all together and just change the cases. Are you trying to save weight?
Always keep the gears and clusters together. Most all the Aluminum cases had the large,308, front bearing. If using the small pilot bellhousing you can't use it. Also watch the cluster shaft bores in the housing, they can get pounded out.