Those are old rear ends. Are any parts hard to get? They are monsters as well. Heavier than many pick up rears. Way overkill for a A body. You'll have more in it to make it work than a good Mopar or Ford rear.
You haven't looked hard or long enough for a 8-3/4. There are dozens of them for sale at Mopar car show/ swap meets. 68original a member here had a complete unit for sale a month ago. As clean as a new one. It sold for around $600. You need to keep looking.
[SOLD] - 8 3/4 rear end - drum to drum