In my part of the country....it would be a lot less expensive to just get another, more narrow rear end than shortening the one you have.....
I had a 55 Chevrolet pickup ( 2 nd series ) ...same basic truck as your 57 and I used a 69 Torino....it was just a bit over 59 inches wide.....measured this way.....
It was almost 3 inches more narrow than the stock rear end....total
It worked great.......it was like having a stock rear that was narrowed 1 and 1/2 inches per side...... I like to run a 3.25 or a 3.50 rear end ratio.....because I usually have tires in the 29 to 30 inch tall range....that's 2700 rpms at 70 with a 30 inch tall tire...anything less.......and the lumpy cammed engine is lugging....
