Only thing is those pesky c-clips.... Sure is easier to set one up than a dana, like those adjustable carrier bearing spanners. Should hold up until you break an axle.
BS on upgrading. I have a friend who has a Demon that had an 8 1/4 in it. Car ran 11s yankin the wheels leavin. He upgraded to an 8 3/4 but not because he broke anything. He was upgrading the whole car. He still has the 8 1/4. They are plenty strong.
I've seen that arguement before too. But in all my years around cars, I have seen precious few c clip buttons broken. They don't hold a load at all. They simply keep the axle in place. It's a good rear end.
I've actually been looking for one for a long time to trade for my complete 8 3/4 sure grip under my 63 Dart, but no one's ever come through. I guess my little slant car will just have overkill, and that's fine too. lol
Only someone standing close to you could answer this question. Or you could sit in a bath tub and smell the bubbles.
If you didn't put a crush sleeve eliminator in that 489, there's your reason. 489s with crush sleeves and 742s usually hold up at those ETs.