There was no holdoff (delay) valve used in A-bodies until '75. The two valves used in the '67 A-body would be the residual-pressure valve and the proportioning valve. The RP valve is located behind the tube seat in the front port of the master cylinder, which feeds the rear (drum) brakes (prior to '67, the RP valve was a separate external unit); its job is to keep a slight amount of pressure in the line when the brakes are released so air won't be sucked into the system past the rear wheel cylinder seals. The need for the RP valve was eliminated in '73 with redesigned rear cylinder seals, and at this late date most drum brake wheel cylinders have the redesigned seals so an RP valve is nowhere near as necessary as it once was. An RP valve must not be present in any port of any master cylinder that feeds disc brakes, because discs will drag if there is any residual pressure in the line.
The proportioning valve is part of the brass combination splitter block/valve assembly you say you swapped from the '68 car. Its job is to reduce pressure to the rear drums because the drums will want to lock up at the line pressure levels required to get effective braking out of the discs up front.
Disc-brake A-bodies did have an endemic tendency to lock the rears prematurely. Everyone griped about it in road tests of the time. Consumer Reports wailed about it. Chrysler did nothing about it, which is dumb because the zero-dollar fix was right on their shelves, see here (and TSB here and MTSC info here). 10" and 11" drum brake wheel cylinders are interchangeable. I routinely swap the 13/16"-bore rear wheel cylinders onto 10" rear drums on cars with 8¾" rear axles, and the ¾"-bore rear wheel cylinders onto 10" rear drums on cars with 7¼" rear axles, and it fixes the problem all the way fixed without having to jack around with an adjustable proportioning valve or otherwise re-engineer the system.
¾" rear wheel cylinder is NAPA 37863 or Raybestos WC-37863 or Bendix
34076 or Wagner WC123412 or Centric 134.67015 or Chrysler 4423 852.
13/16" rear wheel cylinder is NAPA 37696, Raybestos WC37696, Wagner F113704, ACDelco 18E268, Bendix 34064, Chrysler 4313 493, or Centric 134.67013.
(And while you are in there swapping the wheel cylinders, make sure to check carefully for oil-contaminated shoes, which will grab and lock no matter what wheel cylinders you've got)
