rit, I agree with you on this one. the thing is should he run a deeper ring gear to pinion tooth depth ? was the 8 3/4 squared in a jig to begin with? I mean, its been shortened, braced spooled and tooth pattern ran in several positions.this 8 3/4 imo should not be breaking. what kind of bearings you using and are you using aftermarket carrier caps?  

without getting into details how to set the ring and pinion, assumming you already know no parts will interchange between the 657 741 742 and 489 case including the pinions,bearings, and cases but the suregrip will fit any case #. assumming your using and remember the left side is the critical side. hi strength alloy steel axle caps or differential caps if preferred make sure they are machined to the exact tolerances of the older caps. your running a shortened rear, braced and setting rear gears to within tolerances you need a new mechanic who can set it up correctly. that or spend the 2 grand for the ford or more for the dana.probably wasnt even checked in a jig for squareness to begin with lol,I run a tad less hp 50, maybe 75 at most and run an 8 3/4 rear with alloy caps, unbraced standard length A body rear,a strange spool with moser axles and been 9,83 in a 32 3300 A body with no trouble whatsoever.end of story.  

richmond pro gear  

ya on hit the pinion is getting crushed the crush sleeve gets crushed Put a Dana60 with good parts in it and have p1ece of mind. With Abrake and that horsepower im surprised you are getting that life. on every leve you pinion depth gets changed.A 1.34 shin cage be better if you dont wanna swap.  

yes ,I agree 110% on the dana but like I said mines held up well. turned a 9.83 with different gearing and a 3800 launch but I play it safe now and launch at 2500 rpm and a 4.30 gear with just a strange spool .moser axles ,billet caps turned down from the old originals and a standard length A body, nonbraced 8 3/4 ,hahaha, I draw a level line down axles to see if any twisting is occuring by checking after a few runs if the line isnt straight.I just run 3 inch inboard B body spring with a top solid adjustable bar type pinion snubber.minimal wheel lift,simple, but this things been 9.83 with minor mods,4.88 gear,4000 rpm launch.I was then sent to the stands for the rest of the day,hahaha maybe im lucky or maybe im good ,dunno,,, hot,of course crush sleeves get crushed  

I also drew a straight line from axle to axle and see no twist yet.  

your answer lies in the rear suspension grasshopper..  

I have a friend with a road runner who installed a brake and is going through the same problems, it looks like the rear end is being shocked by the trans. brake and riping up the ring gear.
With the kind of horsepower and probably very large tire you are likely running a dana or a Ford 9" is the way to go.