I've got one of these (distribution block and pressure warning switch) with an adjustable proportioning valve on the rear line. I did this because I wanted to keep the warning light. I should also note that I have manual brakes and a modern MC.
You could pipe the MC Front to the two front lines with a Tee and pipe the MC Rear to the adjustable prop valve and that to the rear line.
This style is a combo distribution/warning/proportioning block. Intended for rear drums. I _think_ removing the proportioning valve/spring would make it act like the block above (do research first!!!)