I have a question for all the Mopar GURU's, I am running a narrowed mopar 8 3/4 (489 casting) rearend in a 3000# car making roughly 725HP at the flywheel, 3 speed auto with a brake. I have strange axles and spool, and have the case back-braced, my trouble is I only get about 40-60 passes on a set of ring gears before there are signs of fractureing, or accually the top of the teeth bust off, I have used every type of gear manufacters and currently running richmonds with redline rear lube. I have tried several different settings ( deep, shallow, tight backlash, loose backlash, and dead on) and all combos are giving the same result. Any help or advice in this situation would be greatly appecicated.

-Michael  

recheck pinion depth and check the spool gearface for runout

Why not contact some suppliers and builders? there should be enough businesses out there that deal in rear ends.  they're advertised in hot rod magazines all the time