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

Interesting. We are still thinking on it. Maybe we will end up narrowing it like you. What are you putting your 9 inch in? And if we were to narrow it, how would that interact with a new set of leaf springs? Any fabrication required for that?

Also really the only reason we aren’t too worried about the leaf spring location was because they ran the shackles along the section of the frames that go toward the back of the car. However, we would still need spacers between the dif and and leaf springs to account for the mounting location. The spacers they used were very uneven…
No fabrication necessary on the except to eliminate "whatever" you have there for spring attachment and replace it with stock shackles and front hangers. Mine is a 64 Valiant, which has the same rear dimensions as your car as far as the rear axle. I had the 9" narrowed to about 1" narrower than stock and of course new spring perches installed at the 43" stock A body width.