742 ? 489

So I've got a question about 8 3/4 rear end. What is the difference between 742 c section and a 489 c section, other than the pinion diam. I have a 489 that I was going to use, and have the opportunity to buy the742 which I will. I was also at the local drags this week end (Thunder in the valley) and saw 3 742 cases with blown out ring gears H.P for sure.
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