Have you tried talking to a tranny shop?
I'd double check and make sure the leak is really a problem with the case, as your posting sounds like you are unsure?
Cat iron can be welded...
50's units are not the same as 60-70's units from what I've been told.
tailshaft spline counts are going to be different.
When converting a Saginaw 4spd to a 4spd with OD, they use an OD unit from the later 3spds, not the 50's vintage ones. So, with that in mind, I doubt that you could put the later parts into the earlier case??
