If you're only running 12s the stock axles are ok. Mine held up to many 150 NOS shot passes for over 6 years of racing.
I also ran a 8" in my comet. It was running 12.50's and it took it for several years. The internals were just a run of the mill ford posi center section and 4.11's with stock axles. I still have it complete sitting on the floor of the garage in perfect condition. However when I went to the 408w I switched it up to the 9" detroit locker with mosers.
Hey SeanC, I have that exact same story. Go 9 inch and save all the butt pains. As always just my .02