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that is correct , the 489 case is stronger, but you have top replace the crush sleeve with the bushing kit,

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That may be debatable but I don't think so. MP could have used any case they wanted to pattern their aluminum case after and they chose to use the 742. There is no debate that the 742 has the biggest rear pinion bearing and that makes it a better choice despite the pinion shaft size. Which as I pointed out earlier is a non issue.

Just how much better is a 742 though? I don't know. I'm not an engineer but I don't think it's a significant difference.

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The 742 design may have been MP choice because of no crush sleeve and just maybe better current gear choice. Additionally, the weakest link is always the first item to be solved in design upgrades, and I would venture to guess mopar engineers at the time of the orignal 742 thought a larger pinion shaft dia was needed, maybe for less shaft deflection under high power between the pinion and the ring, which could lead to sheared teeth. With the new alum case reportedably designed 10%? stiffer, going back to the larger bearing non crush sleeve common 742 made the most sense

To add: I don't believe bearing size of the pinion is hardly any part of deflection issues. Roller bearings hardly deflect at all until complete failure. However a pinion load is cantilevered past the rear pinion bearing, allowing for possible deflection of the pinion shaft, a design shortfall the 9" does not have.