I'm rebuilding a 57 chevy truck -   trying to decide whether to keep the original rear-end that has 6 lugs or change out the rear-end and go to a 5 lug pattern. We’re upgrading the front to disc rotors so we can go either way now.  

it just depends on the type of wheels you want to run.  

You should change rear end the one you have is to low geared for todays standerds. Eather that or change gears in the one you have that can be spendy comeparied to a used rear that will com with late model brakes.  

I just changed the stocker to a rear end from an 81 camaro because I did the same thing up front with the disc brakes. A fairly EZ swap but a little welding involved on the new perches, paid a shop that does gear changes and rear ends to install it and hook the brake lines back up for around 300 bucks.
Now one spare tire for both front and rear,shoulda done this years ago, much better driver with that low geared stock rear end gone!