I have the same question. I have a 741 rear with 3.91's. I want to swap to 3.55 sure grip with the big u-joints for my future big block but unsure which carrier to use. The difference is that the 741 has the smallest diameter pinion shaft, 742 is larger diameter, and the 489 is a tapered pinion shaft. Randy's ring and pinon sells the whole shebang for about $1000. I know the 489 is the "correct" chunk for the E bodies, but other than that why is it "preferred" over the 742? Randy's charges more for the 742 than the 489.