Factory 489 8.75 Rear Gear Ringand Pinion?

Anybody know how strong a factory installed 3.91 Gear set from a 489 case 8.75" Mopar rear can be? How much hp/tq can they handle with 35 spline axles/spool @3600lbs? Making over 600 hp/tq....
Are they a strong steel or weak?

Should I just buy after market gears? I already broke the stock axles!

Thanks
Adam