That's a lot of ft/lb on a stock shaft but ,breaking a r/p doesn't help also how was that out put shaft was in that case? We broke the R and P when it had the 580 in it. Stock I/P and O/P at the time, and I'm guessing since the rear jammed, the "shock" broke the O/P shaft at the weakest place. The R and P failed as it was a hardened "street" set of gears. If you look closely you can see virtually all the teeth are cracked, they were just waiting to break. Since we were soon to be fitting a 655 motor in it, we upgraded both I/P and O/P shafts for insurance. We had previously changed the front drum gear carrier after stripping the splines out of the stock aluminium one.It's a pretty strong box and has all of the usual upgrades but if we keep increasing the power we will likely end up with a TH400 (or Rossler) Nearly everyone I know has picked up some et when doing that swap. HTH's 
'74 Challenger..9.46 @ 145.9 1/4, 6.001 @ 118 1/8 so far. 4023lb !!! # N/A, Marsh performance 655ci, Indy Maxx, T/R, Indy 600-13 X's, Street legal, pump gas, full interior, Cal-Tracs, mufflers, 3:73's and real 10.5 radials.
9.51 @ 142.4 1/4, 6.003 @ 114 1/8 with our old mule KB, 572-13, 580 wedge.
RHD '68 Barracuda Fastback 323ci street/strip. Best ET 13.88 @ 99.03
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