For what it's worth, I put a factory original 4.10 Dana 60 in my '70 Roadrunner back in the mid-seventies. I doubt I was much more than 400 HP. The torqueflite I ran was modified and slammed each gear so hard it would 'irk' the tires every time it shifted - even idling with my foot off the pedal.
It was my daily driver and after about 5 years I did break an axle. I don't know how many thousands of times it had shifted during those years but sometimes it must have been 100s of times a day. I guess the HP amount isn't the only factor. Stressed enough times, metal fatigue happens.
