Will an 8 3/4 rear end fit in a 73 dart sport?

Personally, I would find what's wrong with your nine, and fix it. 8 3/4 is not an improvement over a nine, nine parts are EVERYWHERE, and a-body 8 3/4s are ferociously expensive (and have the wrong bolt pattern for you). Early b-body could work, but the nine is already there.
Just as an aside. With both rear wheels off the ground, when you manually spin one rear tire, what does the other rear tire do?
Also: are you dead certain its a nine inch, and not an eight inch?