In response to the "ring gear is deflecting" train of thought, of course it deflects some. but I believe it's more pinion deflection. Basically the major thing the Ford 9" has over the 8 3/4 or 9 1/4 or 12-bolt is the pinion support. That's what seems to me to be making the difference between 8 3/4 and 9".
My friend with the '63 Corvette broke countless 12 bolts before switching to a Dana 60 and hasn't broken anything since.
