They never made a posi for a 57 Pontiac but there are some very rare 58's with a posi. Your rear has the 29 spline axles which is the problem. In 59 thru 64 they used 31 spline axles and there are quite a few posi center sections out there but you would need to buy the correct length 31 spline axles. Fabcraft in Texas can help you. These are not cheap rears but they are very strong.
Now if the pontiac is a canadian built pontiac it would have the chevy rear end and eaton would have a bolt in posi for it for around 600 they may even have one for the american pontiac but I know for sure the canadian pontiac has the same rear as the chevy and the bolt in eaton will work. Really no way to know though if you only have the rear end.