Brake systems of that era can be different than later stuff, I daily drive a 51 Plymouth so I understand.

I also understand the hacks prior owners can inflict on old cars. You say the brakes have always bee iffy and the one wheel has always locked up.

You say that you've pretty much replaced everything, except hard lines.

I know nothing about the braking system on a 57 Chevy so I started googling. Lots of disc brake swap kits came up, damn near zero "how to rebuild the stock rear drum" info though.

The problem I see is how do you know it's put together correctly? If you put it together the way it came apart you may have just reinstalled the problem only with new parts.

Here's a shot of the rear brakes from trifive.com for reference. Key thing to remember that the parts are installed in reference to the front of the car, not your left or right. For example, the ebrake lever will be on the rear of the assembly which means on one side it will be on your right, but on the other it'll be on your left. All parts will be like that. Sorry to be so basic but that will sometimes bite even the best of us.