Get one From a 67 disk/drum car ( or drum-drum depending on final configuration) And buy a few bucks in fittings. You need to isolate the rear circuit and bring up to M/c or combo block. Front lines plumb into combo valve on their own fittings.
I recently converted my '62 Dart (B-body) from single drum-drum to dual drum-drum. As above: the '67 MC worked fine with a few parts from Inline Tube. Pedal rod did not change and it bolts right in on the firewall. It is a good safety upgrade, IMO.