We've broken an O/P shaft, at the time the car was making around 880hp and weighed 3950ish (w/d). We had just put a new set of slicks on. We upgraded both the i/p and o/p shafts while it was apart. We got round the speedo issue going to a Dakota Dig dash with a GPS speedo.
Yikes! Killed the sprag, too! It did !! You can see the weak point on the shaft was the oil feed. I also heard you can run 904 internals in a 727 case and that they are maintenance intensive, the reason to do it is likely to be it takes less H/P to run. A lot of the stock eliminator / super stock folk run them I'm told. A and A also do different ratios for the 727 but only 4 or 5, I was looking at a 2.28 1st gear set but it was cheaper to change the ratio in the rear (Dana 60). Try as I may, I could not find a 3.73 Ratio for a Dana in a pro gear and ended up using a street gear but that's another story. 
'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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