I would bet that unless it was an all out gasser that the stock, Junior stock and Modified Production cars were running thier stock rear ends. Tire technology wasn't as it is today and the bite wasn't as good so the tires would slip more, lessening the shock on the differentials.
my personal experiance, the left side axle would break after about a dozen street races with wide STREET tires. replaced it about 5 times, (the "yard guy" knew what i needed after the second one.....), so i put a 9" ford in the car.
I put a '56 Olds rearend in my '57 Chevie back in the early 70's. I ran a 350hp 327 for years. The Olds rear end held up well. The Olds rearend is still in the car, but I'm planning on replacing it with a '57 chevie posi.
M/P cars could all use whatever axle they wanted as far as I know, certainly that was the case in the 70s when I raced. But by then the 7" tire rule was gone too, so there may have been some evolution in the rest of the rules too.
I've got a book called "Gasser Wars" that has lots of really good pictures and some really interesting information. There were lots of guys in gas classes that appear to run truck rear ends - in the photo's you can see six bolt wheel patterns. Of course, there were some guys running quick change rears too...
The stock 55-57 rears were weak and easy to break.
In the mid 60s a popular rear end swap was the olds/pontiac rears.
My first 57, a 210 2 dr post had a studebaker truck rear end with 4.88s when I got it. Only problem was it used 5X5 bolt pattern and the large retaining nut took about a 6 foot cheater bar to get it off.
Early 50s studebaker trucks with overdrive had 4.88s and the ujoint is the same as is the spring perch mounts and the width. A very easy swap that I did for several friends who wanted cheap low gears for drag racing, we used to give $10 for the rears if we took them out.
Never saw one break.
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