Shane - A little searching for more details about the Kent Moore J-3289 can be seen below in the extract pic from a
Kent Moore Fall 1974 Catalog.
If you're wanting to stick with the original Kent Moore tools, you need the J-3289-01 differential holder kit, which is supposed to include the J-3289-20 mount, as well as the J-8622 fixture adapter kit (not originally part of the J-3289-01 kit). On the fixture adapter kit, you may be able to just fabricate some plates to take the place of the adapter plates to hold your HO52 differential.
Wish you luck in finding what you need.
