I installed a Wagner MC71258 dual master cylinder. I believe what I need is 4 residual pressure valves to maintain 10 pounds of pressure on each wheel cylinder.
I just googled it and it came up with 2 different options
option 1 was power drum brakes /w 1 inch bore
option 2 was manual drum brakes with 1 inch bore 67-68
but that only showed up in pre 69 models.
you could have a power drum brake master cylinder.