Those rear ends are "bullet' proof!!! Used a 57 Olds [virtually the same] on a T- Bucket back in the early 70's & put one in a 55 chevy I had, to stop the Grenading stock rr. end!!!!
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I can only please one person a day, Today is not your day!!Tomorrow doesn't look good either !!!!
A 57' pontiac rear is a bolt in to a 57 chev and much stronger. The only weak areas are the housing itself is pretty thin and will tear the spring mounts out on a serious drag car (just weld on some gussets) and the 57' axles are an oddball spline count, so only 57' center sections are a direct swap. You can get axles for them with the more common later spline count that will let you swap in any later center section.
The ad you responded to was posted in 2015. The seller/poster has not been on this site since April of 2017.
That being said, his profile shows that he is located in or near Parry Sound and his full name is Patrick James Christie.
A quick search of Canada411 shows several residents of Parry Sound with that last name. Your best bet of getting in touch with Patrick may be by contacting the Christies in Parry Sound and asking if the can provide contact info for Patrick James.
Just a suggestion, hope it works out for you.
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Actually, in real racing ... it IS how fast you went.