Mike, the web site I previously posted is selling brand new Delco 50DN alternators, your configuration cheaper than I can get one rebuilt and they do not require a core. That's where I got my new alternator. They call them Elreg alternators, but as I posted previously, they are made in Canada and they have Delco Remy cast into the parts.
If you alternator is working and all that is leaking is the main post, I agree with Joe and would just repair that. I would get a gasket kit because when the rear cover is removed I suspect the gasket that is on there will not seal as good as before.