1974 Dart Sport 489 Differential Question

No A Body after 72 ever had a factory installed 8 3/4 this was also the same year they got rid of Small Bolt Pattern. (except some 73 Slant 6 Stripper cars which had 4 wheel drums)
So If it is Big Bolt Pattern the axels were swaped or drilled out

1974 with a slant 6 or 318 would have been a 7 1/4 or 8 1/4 from the factory
8 1/4 being an added cost option