I installed a 1981 Trans am disc brake rear end in mine but Im not sure if there's a salvage yard swap that would work on your Chevy II rear end there's some really cool conversion kits out there. I'm also running a firewall mounted booster/master cylinder on my 51 and yes you will have to get a disc/disc proportioning valve,
Drum Brakes require residual pressure of 10 lbs, The disc/disc setup does not require this pressure. They only require 2 lbs of pressure

But I don't think you'll need to get the residual pressure valves They run in-line and act as a one way check valve and prevent fluid from bleeding back to the master cylinder. They are usually only installed when mounting your master cylinder under the floor
you shouldn't have the problem with fluid bleeding back to the master cylinder on the firewall mounted booster/master cylinder

Todd

Some people are like Slinkies, Their not really good for nothing... But they still bring a smile to your face when you push them down the stairs.

1951 3100 350 TBI Gallery Toddzilla... "$old"