[RichardJ]

>>Which side of the prop valve is for the front brakes, which for the rear?<<

The rear outlet of the Combo Valve is the Proportioning end. The front two outlets are the Metering Valve and they are for the front brakes.

>>Is the disc brake circuit on a '71 MC the one closest to the firewall? or to the front of the car?<<

Many, but not all disc/drum MC have a larger reservoir for the disc brake outlet, but the important thing is that the pressure from both MC outlets is exactly the same. Technically, either MC outlet can be used for the disc calipers. The size of the reservoir only becomes important when the disc pads wear down to nothing and the reserve capacity of the reservoir is exceeded and it starts pumping air.

Your summit listing shows the Metering valve outlets. You should be able to follow those lines to the front brakes.