What is the difference in a 741 and a 742 8 3/4 case?

[moparmarks]

Yep your right, my bad. 1-3/8". For some reason I thought 1-5/8"

 

[gtxracer]

Biggest (and probably the only) difference is the size of the pinion shaft. The 741 is the smallest of the three and the 742 is the largest. The 489 sits in the middle.

In the tech archives on Moparts there's a comparison photo of all the popular rear ends including the Dana, GM 12 bolt and the 9" Ford.

Stanton, I think you might be mistaken, the 742 has a 1.75 pinion and the 489 has a 1.875 pinion, or maybe I am wrong.

 

[gtxracer]

Currently I have a 742 w/4.88 gears in my GTX and a spare 489 w/4.56 with a solid spacer, both with spools. My '65 Dodge Coronet street ride also has a 742 with 4.30 gears and a clutch type posi and I have never had an issue with either.