If all you are doing is updating the MC then stick with factory stuff. Get a later dual reservoir manual MC and it'll bolt on.

You'll likely need a T splitter to tie both the front lines to the rear port and add a bit of hard line to tie the rear brakes to the front port. I suppose you could run a factory splitter from a later all drum setup as well, CI sells one new.

That's exactly how I update my 64 Chrysler to dual reservoir, though in my case it was a power setup.