The rear end under your car originally was in the 59 inch range. Measured this way ...
I have used 57 to 59 Ford passenger car rear ends under the 49 to 54 Chevrolet cars ... but then ...
I use them under almost everything I do ... 78 up Monte Carlo and Malibu rears also work nice ... and have the standard GM Chevrolet wheel pattern ( 5 on 4.5 ) .
Here is a link that lists a good many vehicle widths ... both front and rear ... for common American vehicles .
http://www.quickperformance.com/Tech...us_measure.htm