742 ? 489

Both cases are pretty strong. The 742 has a larger rear pinion bearing than the 489, and doesnt require a crush sleeve (the 489 can use a solid spacer and shims though if needed). Some low cost ring and pinion sets are not made for more than street use, and cannot take as much racing abuse. If you need 100% bullitt-proof, a Dana rear is the way to go.

Here is a good comparison of the pinions in those cases:
http://www.moparts.org/Tech/Archive/axle/1.html