You'll need at least the metering valve which delays the fronts slightly until the drums can engage. That gives you more even braking. Also possibly the prop valve but who knows since GM installed it in all kinds of combos. I've got an orignal four wheel manual drum car and an original front power disc car. Both of them had it.
If you don't have anything I'd recommend the PV71 that most places carry. It's got your distribution/metering/proportional all in one unit. Functions the same as the factory setup was meant to, they just started putting everything in one package in the 70's. I can't say how it would work if you had the prop valve under the door and left it there though. You may need to do some line work based on what's in there now if you were to go with that combo valve.
