building 8 3/4 489 section

According to my time slip 375 to the wheels and I don't think - I know it broke. Brand new from dr. Diff ring and pinion spool and bearings. set up by the most reputable rear end shop here in Portland. And I added that to get to the quick about the next two questions were they old gears and did I set them up myself.
So how many horses do you think broke your 8 3/4?