I ran a '63 Factory Posi Unit in my '56 Nomad for about the 8 last years I had it. Had a 327 pushing about 325 HP. Had about 100 Hi-Rev Burnouts and about 500 true Power Shifts (pedal to the floor and stayed there) on it with the 4-Speed Muncie and Hurst Comp Plus Shifter, spinning wide H-60 Tires to boot.
The Rear never wimpered. Was just as good as the day I installed it.
But that was my car.
A friend of mine also ran a Factory Posi in his '57 Bel-Air with a built 402 Big Block, 4-Speed Rock Crusher. I'm sure it had at least 450 HP. He didn't do quite as much racing as I did, but same story, Rear never wimpered. One day he blew the Rear Universal to smithereens, but never the Rear.
-Parting Out over 75 '58 to '73 Full Size Chevy Cars-
-National Impala Association
-Vintage Chevrolet Club of America