Proportioning valve location

A two-port valve needs to be the last item in the line on the way to the rear end.
It doesn't matter where you physically install it, so long as it remains bleedable, and the outlet goes to the rear. In theory you could even mount it at the back of the car.

If you have a dual M/C and your distribution block has a safety lite in it, and you put this valve ahead of it, the light will go on every time that you lean on the pedal.
If you do not have a dual M/C, then you do not have this pressure-differential valve. But the new valve still has to be last in line.
Willwood makes a nice combination valve that mounts up high, near or on, the M/C.