I kinda go by what a brilliant mopar maven, Monte Smith said about 8 3/4. He said, basically and paraphrased, the 8 3/4 case is the weak point, allowing the ring to walk away from the pinion, and shear teeth. At what point that happens, depends on the parts involved ( nodular case and spool, or stock case and sg, etc.) Bracing the housing can help, just like it does with a nine, but the case is the weak point. The nine now has a bunch of much stronger cases than stock, but the original 9" grey iron cases were certainly no better than an 8 3/4 case, thats why original nodular ford cases were so prized/expensive.
I have had no problems with a 500hp 440 in a b-body with my 8 3/4, using slicks and a 4000 converter. But i still have stock axles, and rather than put lots more money in my 8 3/4, i would put in a dana.
If you are looking at 600 hp with a stick, or transbrake, personally i would step up to a dana or nine. The money spent to upgrade a 8 3/4 to handle that power would be better spent on something else.
