Welcome! This is as good of a place as any to post, so why not...
I haven't heard of using a Caddy rear end before. As far as I know, there are really only 2 considerations you need to be worried about: lug spacing and track width.
The '57 had 6 lugs originally, didn't it? (Can't remember.) If you're happy with carrying around 2 spare tires, or converting the front to 5-lug too, then that's no problem.
As for the width, grab a tape measure and see how wide each is. If it's close, it's a good fit.
You will need to weld new spring perches onto the axle (available at an alignment shop) and also you may need to modify the driveshaft (lengthen, shorten, or change ends).
No ideas about the e-brake, is it mechanical or hydraulic?
-Boyo
